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

deps/deps.go

@@ -37,8 +37,8 @@ type Deps struct {
 	// Used to log warnings that may repeat itself many times.
 	DistinctWarningLog *helpers.DistinctLogger
 
-	// The templates to use. This will usually implement the full tpl.TemplateHandler.
-	Tmpl tpl.TemplateFinder `json:"-"`
+	// The templates to use. This will usually implement the full tpl.TemplateManager.
+	Tmpl tpl.TemplateHandler `json:"-"`
 
 	// We use this to parse and execute ad-hoc text templates.
 	TextTmpl tpl.TemplateParseFinder `json:"-"`
@@ -77,7 +77,10 @@ type Deps struct {
 	OutputFormatsConfig output.Formats
 
 	templateProvider ResourceProvider
-	WithTemplate     func(templ tpl.TemplateHandler) error `json:"-"`
+	WithTemplate     func(templ tpl.TemplateManager) error `json:"-"`
+
+	// Used in tests
+	OverloadedTemplateFuncs map[string]interface{}
 
 	translationProvider ResourceProvider
 
@@ -151,8 +154,8 @@ type ResourceProvider interface {
 }
 
 // TemplateHandler returns the used tpl.TemplateFinder as tpl.TemplateHandler.
-func (d *Deps) TemplateHandler() tpl.TemplateHandler {
-	return d.Tmpl.(tpl.TemplateHandler)
+func (d *Deps) TemplateHandler() tpl.TemplateManager {
+	return d.Tmpl.(tpl.TemplateManager)
 }
 
 // LoadResources loads translations and templates.
@@ -239,24 +242,25 @@ func New(cfg DepsCfg) (*Deps, error) {
 	distinctWarnLogger := helpers.NewDistinctLogger(logger.WARN)
 
 	d := &Deps{
-		Fs:                  fs,
-		Log:                 logger,
-		DistinctErrorLog:    distinctErrorLogger,
-		DistinctWarningLog:  distinctWarnLogger,
-		templateProvider:    cfg.TemplateProvider,
-		translationProvider: cfg.TranslationProvider,
-		WithTemplate:        cfg.WithTemplate,
-		PathSpec:            ps,
-		ContentSpec:         contentSpec,
-		SourceSpec:          sp,
-		ResourceSpec:        resourceSpec,
-		Cfg:                 cfg.Language,
-		Language:            cfg.Language,
-		Site:                cfg.Site,
-		FileCaches:          fileCaches,
-		BuildStartListeners: &Listeners{},
-		Timeout:             time.Duration(timeoutms) * time.Millisecond,
-		globalErrHandler:    &globalErrHandler{},
+		Fs:                      fs,
+		Log:                     logger,
+		DistinctErrorLog:        distinctErrorLogger,
+		DistinctWarningLog:      distinctWarnLogger,
+		templateProvider:        cfg.TemplateProvider,
+		translationProvider:     cfg.TranslationProvider,
+		WithTemplate:            cfg.WithTemplate,
+		OverloadedTemplateFuncs: cfg.OverloadedTemplateFuncs,
+		PathSpec:                ps,
+		ContentSpec:             contentSpec,
+		SourceSpec:              sp,
+		ResourceSpec:            resourceSpec,
+		Cfg:                     cfg.Language,
+		Language:                cfg.Language,
+		Site:                    cfg.Site,
+		FileCaches:              fileCaches,
+		BuildStartListeners:     &Listeners{},
+		Timeout:                 time.Duration(timeoutms) * time.Millisecond,
+		globalErrHandler:        &globalErrHandler{},
 	}
 
 	if cfg.Cfg.GetBool("templateMetrics") {
@@ -344,7 +348,9 @@ type DepsCfg struct {
 
 	// Template handling.
 	TemplateProvider ResourceProvider
-	WithTemplate     func(templ tpl.TemplateHandler) error
+	WithTemplate     func(templ tpl.TemplateManager) error
+	// Used in tests
+	OverloadedTemplateFuncs map[string]interface{}
 
 	// i18n handling.
 	TranslationProvider ResourceProvider