Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

tpl/tplimpl/subcategory_string.go

@@ -0,0 +1,25 @@
+// Code generated by "stringer -type SubCategory"; DO NOT EDIT.
+
+package tplimpl
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[SubCategoryMain-0]
+	_ = x[SubCategoryEmbedded-1]
+	_ = x[SubCategoryInline-2]
+}
+
+const _SubCategory_name = "SubCategoryMainSubCategoryEmbeddedSubCategoryInline"
+
+var _SubCategory_index = [...]uint8{0, 15, 34, 51}
+
+func (i SubCategory) String() string {
+	if i < 0 || i >= SubCategory(len(_SubCategory_index)-1) {
+		return "SubCategory(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _SubCategory_name[_SubCategory_index[i]:_SubCategory_index[i+1]]
+}