Rename AST structures to Tree and relocate freeing logic

v0/ast/expression.c

@@ -0,0 +1,9 @@
+#include "expression.h"
+#include <stdlib.h>
+
+void ast_free_type(TypeTree* expr) {
+    if (expr->tag == TYPE_TREE_ARRAY) {
+        ast_free_type(expr->array.array);
+        free(expr->array.array);
+    }
+}