Move direct superclass checking to `test/t/superclass.rb`.

[minor-fix]Update value pointer comments

Implement String#slice!

Move mrbgems ISO direct superclass test to `superclass.rb`.
Skips test if class isn't defined.
Close #2332.

Change to always use locals->car->car.

Fix clang analizer warnings

Fix conflict doc since there is only `add_conflict` method.

* master:
  update add_confict document; ref #2391
  Update document by regenerating it.
  Add :doc task to run document generator.
  Add document for `add_conflict` and `add_conflicts`.

Update document and add :doc task.

This reverts commit 29f14e72.

Fix memory leaks in String

Refactor how to use RSTR_NOFREE_P macro