Add creating tree functions.

src/tree.c

@@ -20,7 +20,7 @@ int libab_tree_has_string(libab_tree_variant variant) {
 int libab_tree_has_scope(libab_tree_variant variant) {
     return variant == TREE_BASE || variant == TREE_BLOCK ||
            variant == TREE_IF || variant == TREE_WHILE ||
-           variant == TREE_DOWHILE || variant == TREE_FUN;
+           variant == TREE_DOWHILE;
 }
 
 int libab_tree_has_type(libab_tree_variant variant) {