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/category_string.go

@@ -0,0 +1,30 @@
+// Code generated by "stringer -type Category"; 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[CategoryLayout-1]
+	_ = x[CategoryBaseof-2]
+	_ = x[CategoryMarkup-3]
+	_ = x[CategoryShortcode-4]
+	_ = x[CategoryPartial-5]
+	_ = x[CategoryServer-6]
+	_ = x[CategoryHugo-7]
+}
+
+const _Category_name = "CategoryLayoutCategoryBaseofCategoryMarkupCategoryShortcodeCategoryPartialCategoryServerCategoryHugo"
+
+var _Category_index = [...]uint8{0, 14, 28, 42, 59, 74, 88, 100}
+
+func (i Category) String() string {
+	i -= 1
+	if i < 0 || i >= Category(len(_Category_index)-1) {
+		return "Category(" + strconv.FormatInt(int64(i+1), 10) + ")"
+	}
+	return _Category_name[_Category_index[i]:_Category_index[i+1]]
+}