Introduce a tree map for all content

This commit introduces a new data structure to store pages and their resources.

This data structure is backed by radix trees.

This simplies tree operations, makes all pages a bundle,  and paves the way for #6310.

It also solves a set of annoying issues (see list below).

Not a motivation behind this, but this commit also makes Hugo in general a little bit faster and more memory effective (see benchmarks). Especially for partial rebuilds on content edits, but also when taxonomies is in use.

```
name                                   old time/op    new time/op    delta
SiteNew/Bundle_with_image/Edit-16        1.32ms ± 8%    1.00ms ± 9%  -24.42%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16    1.28ms ± 0%    0.94ms ± 0%  -26.26%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      33.9ms ± 2%    21.8ms ± 1%  -35.67%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            40.6ms ± 1%    37.7ms ± 3%   -7.20%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        56.7ms ± 0%    51.7ms ± 1%   -8.82%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      19.9ms ± 2%    18.3ms ± 3%   -7.64%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         37.9ms ± 4%    34.0ms ± 2%  -10.28%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             10.7ms ± 0%    10.6ms ± 0%   -1.15%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         10.8ms ± 0%    10.7ms ± 0%   -1.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16           43.2ms ± 1%    39.6ms ± 1%   -8.35%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 47.6ms ± 1%    47.3ms ± 0%     ~     (p=0.057 n=4+4)
SiteNew/Deep_content_tree-16             73.0ms ± 1%    74.2ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Many_HTML_templates-16           37.9ms ± 0%    38.1ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Page_collections-16              53.6ms ± 1%    54.7ms ± 1%   +2.09%  (p=0.029 n=4+4)

name                                   old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image/Edit-16         486kB ± 0%     430kB ± 0%  -11.47%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     265kB ± 0%     209kB ± 0%  -21.06%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      13.6MB ± 0%     8.8MB ± 0%  -34.93%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            66.5MB ± 0%    63.9MB ± 0%   -3.95%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        28.8MB ± 0%    25.8MB ± 0%  -10.55%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      6.16MB ± 0%    5.56MB ± 0%   -9.86%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         16.9MB ± 0%    16.0MB ± 0%   -5.19%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             2.28MB ± 0%    2.29MB ± 0%   +0.35%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         2.07MB ± 0%    2.07MB ± 0%     ~     (p=0.114 n=4+4)
SiteNew/Tags_and_categories-16           14.3MB ± 0%    13.2MB ± 0%   -7.30%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 69.1MB ± 0%    69.0MB ± 0%     ~     (p=0.343 n=4+4)
SiteNew/Deep_content_tree-16             31.3MB ± 0%    31.8MB ± 0%   +1.49%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16           10.8MB ± 0%    10.9MB ± 0%   +1.11%  (p=0.029 n=4+4)
SiteNew/Page_collections-16              21.4MB ± 0%    21.6MB ± 0%   +1.15%  (p=0.029 n=4+4)

name                                   old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image/Edit-16         4.74k ± 0%     3.86k ± 0%  -18.57%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     4.73k ± 0%     3.85k ± 0%  -18.58%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16        301k ± 0%      198k ± 0%  -34.14%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16              389k ± 0%      373k ± 0%   -4.07%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16          338k ± 0%      262k ± 0%  -22.63%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16        102k ± 0%       88k ± 0%  -13.81%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16           176k ± 0%      152k ± 0%  -13.32%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16              26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16          26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16             273k ± 0%      245k ± 0%  -10.36%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                   396k ± 0%      398k ± 0%   +0.39%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16               317k ± 0%      325k ± 0%   +2.53%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16             146k ± 0%      147k ± 0%   +0.98%  (p=0.029 n=4+4)
SiteNew/Page_collections-16                210k ± 0%      215k ± 0%   +2.44%  (p=0.029 n=4+4)
```

Fixes #6312
Fixes #6087
Fixes #6738
Fixes #6412
Fixes #6743
Fixes #6875
Fixes #6034
Fixes #6902
Fixes #6173
Fixes #6590

2 files changed, 72 insertions, 0 deletions

diff --git a/htesting/hqt/checkers.go b/htesting/hqt/checkers.go
index eb8b733ac..6fb65ee47 100644
--- a/htesting/hqt/checkers.go
+++ b/htesting/hqt/checkers.go
@@ -15,12 +15,24 @@ package hqt
 
 import (
 	"errors"
+	"fmt"
 	"reflect"
+	"strings"
 
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/htesting"
 	"github.com/google/go-cmp/cmp"
+	"github.com/spf13/cast"
 )
 
+// IsSameString asserts that two strings are equal. The two strings
+// are normalized (whitespace removed) before doing a ==.
+// Also note that two strings can be the same even if they're of different
+// types.
+var IsSameString qt.Checker = &stringChecker{
+	argNames: []string{"got", "want"},
+}
+
 // IsSameType asserts that got is the same type as want.
 var IsSameType qt.Checker = &typeChecker{
 	argNames: []string{"got", "want"},
@@ -47,6 +59,36 @@ func (c *typeChecker) Check(got interface{}, args []interface{}, note func(key s
 	return nil
 }
 
+type stringChecker struct {
+	argNames
+}
+
+// Check implements Checker.Check by checking that got and args[0] represents the same normalized text (whitespace etc. trimmed).
+func (c *stringChecker) Check(got interface{}, args []interface{}, note func(key string, value interface{})) (err error) {
+	s1, s2 := cast.ToString(got), cast.ToString(args[0])
+
+	if s1 == s2 {
+		return nil
+	}
+
+	s1, s2 = normalizeString(s1), normalizeString(s2)
+
+	if s1 == s2 {
+		return nil
+	}
+
+	return fmt.Errorf("values are not the same text: %s", htesting.DiffStrings(s1, s2))
+}
+
+func normalizeString(s string) string {
+	lines := strings.Split(strings.TrimSpace(s), "\n")
+	for i, line := range lines {
+		lines[i] = strings.TrimSpace(line)
+	}
+
+	return strings.Join(lines, "\n")
+}
+
 // DeepAllowUnexported creates an option to allow compare of unexported types
 // in the given list of types.
 // see https://github.com/google/go-cmp/issues/40#issuecomment-328615283
diff --git a/htesting/test_helpers.go b/htesting/test_helpers.go
index 660c76a44..3804f28fe 100644
--- a/htesting/test_helpers.go
+++ b/htesting/test_helpers.go
@@ -56,3 +56,33 @@ var rnd = rand.New(rand.NewSource(time.Now().UnixNano()))
 func RandIntn(n int) int {
 	return rnd.Intn(n)
 }
+
+// DiffStringSlices returns the difference between two string slices.
+// Useful in tests.
+// See:
+// http://stackoverflow.com/questions/19374219/how-to-find-the-difference-between-two-slices-of-strings-in-golang
+func DiffStringSlices(slice1 []string, slice2 []string) []string {
+	diffStr := []string{}
+	m := map[string]int{}
+
+	for _, s1Val := range slice1 {
+		m[s1Val] = 1
+	}
+	for _, s2Val := range slice2 {
+		m[s2Val] = m[s2Val] + 1
+	}
+
+	for mKey, mVal := range m {
+		if mVal == 1 {
+			diffStr = append(diffStr, mKey)
+		}
+	}
+
+	return diffStr
+}
+
+// DiffStrings splits the strings into fields and runs it into DiffStringSlices.
+// Useful for tests.
+func DiffStrings(s1, s2 string) []string {
+	return DiffStringSlices(strings.Fields(s1), strings.Fields(s2))
+}