tpl: Add templates.Current

This commit also

* Unexport all internal state in TemplateInfo.
* Make the dispatcher keys used for passing context.Context into uint8 from string to save memory allocations.

Co-authored-by: Joe Mooring <joe@mooring.com>

Updates #13571

tpl/templates/templates_integration_test.go

@@ -14,6 +14,7 @@
 package templates_test
 
 import (
+	"path/filepath"
 	"testing"
 
 	"github.com/gohugoio/hugo/hugolib"
@@ -127,3 +128,41 @@ Try printf: {{ (try (printf "hello %s" "world")).Value }}
 		"Try printf: hello world",
 	)
 }
+
+func TestTemplatesCurrent(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/baseof.html --
+baseof: {{ block "main" . }}{{ end }}
+-- layouts/all.html --
+{{ define "main" }}
+all.current: {{ templates.Current.Name }}
+all.current.filename: {{ templates.Current.Filename }}
+all.base: {{ with templates.Current.Base }}{{ .Name }}{{ end }}|
+all.parent: {{ with .Parent }}Name: {{ .Name }}{{ end }}|
+{{ partial "p1.html" . }}
+{{ end }}
+-- layouts/_partials/p1.html --
+p1.current: {{ with templates.Current }}Name: {{ .Name }}|{{ with .Parent }}Parent.Name: {{ .Name }}{{ end }}{{ end }}|
+p1.current.Ancestors: {{ with templates.Current }}{{ range .Ancestors }}{{ .Name }}|{{ end }}{{ end }}
+{{ partial "p2.html" . }}
+-- layouts/_partials/p2.html --
+p2.current: {{ with templates.Current }}Name: {{ .Name }}|{{ with .Parent }}Parent.Name: {{ .Name }}{{ end }}{{ end }}|
+p2.current.Ancestors: {{ with templates.Current }}{{ range .Ancestors }}{{ .Name }}|{{ end }}{{ end }}
+p3.current.Ancestors.Reverse: {{ with templates.Current }}{{ range .Ancestors.Reverse }}{{ .Name }}|{{ end }}{{ end }}
+
+`
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html",
+		"all.current: all.html",
+		filepath.FromSlash("all.current.filename: /layouts/all.html"),
+		"all.base: baseof.html",
+		"all.parent: |",
+		"p1.current: Name: _partials/p1.html|Parent.Name: all.html|",
+		"p1.current.Ancestors: all.html|",
+		"p2.current.Ancestors: _partials/p1.html|all.html",
+	)
+}