Add p_context_delete() and p_tree_delete() for D targets

spec/test_tree_field_aliases.d

@@ -10,12 +10,13 @@ int main()
 unittest
 {
     string input = "\na\nb\nc";
-    p_context_t * context;
-    context = p_context_new(input);
+    p_context_t * context = p_context_new(input);
     assert(p_parse(context) == P_SUCCESS);
     Start * start = p_result(context);
 
     assert_eq(TOKEN_a, start.first.pToken.token);
     assert_eq(TOKEN_b, start.second.pToken.token);
     assert_eq(TOKEN_c, start.third.pToken.token);
+
+    p_tree_delete(start);
 }