Rework template handling for function and map lookups

This is a big commit, but it deletes lots of code and simplifies a lot.

* Resolving the template funcs at execution time means we don't have to create template clones per site
* Having a custom map resolver means that we can remove the AST lower case transformation for the special lower case Params map

Not only is the above easier to reason about, it's also faster, especially if you have more than one language, as in the benchmark below:

```
name                          old time/op    new time/op    delta
SiteNew/Deep_content_tree-16    53.7ms ± 0%    48.1ms ± 2%  -10.38%  (p=0.029 n=4+4)

name                          old alloc/op   new alloc/op   delta
SiteNew/Deep_content_tree-16    41.0MB ± 0%    36.8MB ± 0%  -10.26%  (p=0.029 n=4+4)

name                          old allocs/op  new allocs/op  delta
SiteNew/Deep_content_tree-16      481k ± 0%      410k ± 0%  -14.66%  (p=0.029 n=4+4)
```

This should be even better if you also have lots of templates.

Closes #6594

1 files changed, 19 insertions, 3 deletions

diff --git a/langs/language.go b/langs/language.go
index 67cb3689a..0e04324e9 100644
--- a/langs/language.go
+++ b/langs/language.go
@@ -16,6 +16,7 @@ package langs
 import (
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/config"
@@ -56,7 +57,9 @@ type Language struct {
 
 	// These are params declared in the [params] section of the language merged with the
 	// site's params, the most specific (language) wins on duplicate keys.
-	params map[string]interface{}
+	params    map[string]interface{}
+	paramsMu  sync.Mutex
+	paramsSet bool
 
 	// These are config values, i.e. the settings declared outside of the [params] section of the language.
 	// This is the map Hugo looks in when looking for configuration values (baseURL etc.).
@@ -123,7 +126,15 @@ func (l Languages) Less(i, j int) bool {
 func (l Languages) Swap(i, j int) { l[i], l[j] = l[j], l[i] }
 
 // Params retunrs language-specific params merged with the global params.
-func (l *Language) Params() map[string]interface{} {
+func (l *Language) Params() maps.Params {
+	// TODO(bep) this construct should not be needed. Create the
+	// language params in one go.
+	l.paramsMu.Lock()
+	defer l.paramsMu.Unlock()
+	if !l.paramsSet {
+		maps.ToLower(l.params)
+		l.paramsSet = true
+	}
 	return l.params
 }
 
@@ -163,7 +174,12 @@ func (l Languages) IsMultihost() bool {
 // SetParam sets a param with the given key and value.
 // SetParam is case-insensitive.
 func (l *Language) SetParam(k string, v interface{}) {
-	l.params[strings.ToLower(k)] = v
+	l.paramsMu.Lock()
+	defer l.paramsMu.Unlock()
+	if l.paramsSet {
+		panic("params cannot be changed once set")
+	}
+	l.params[k] = v
 }
 
 // GetBool returns the value associated with the key as a boolean.