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Update mrbconf document.

Adapt a behavior of String#slice! to other Ruby implementations

Start `mrbconf` documentation.

Add fixed state atexit stack feature.

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

[minor-fix]Update value pointer comments

Implement String#slice!

Adds following macros:
* MRB_FIXED_STATE_ATEXIT_STACK (not defined by default)
  * When defined enables fixed state atexit stack.
* MRB_FIXED_STATE_ATEXIT_STACK_SIZE (default value: 5)
  * This macro will be ignored when `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined.
  * When `mrb_state_atexit` is called more than this value it will raise runtime error.

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.