feat(ast): ElementKind list to keep track of all possible ast elements

This is will be useful for being able to marshal and unmarshal the ast,
since we can't easily know what implementation of the Element interface
is supposed to be used.

ipub/ast/ast.go

@@ -186,3 +186,11 @@ func ensureIsolated(e Element) {
 
 type ElementKind string
 
+var elementKindList = map[ElementKind]Element{}
+
+func NewElementKind(name string, t Element) ElementKind {
+	k := ElementKind(name)
+	elementKindList[k] = t
+
+	return k
+}