Change to use assert method.
Move direct superclass checking to `test/t/superclass.rb`.
Test all ISO defined classes direct superclass except `Object` class.
Move mrbgems ISO direct superclass test to `superclass.rb`.
Skips test if class isn't defined.
Close #2332.
Implement String#slice_bang
Unsupported Regex in mrb_str_aref_m()
Add fixed state atexit stack feature.
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.
Merge pull request #2397 from suzukaze/add-string-slice_bang

Implement String#slice!
[minor-fix]Minor cleanups on variable.c comments
Merge pull request #2402 from katmutua/master

[minor-fix]Update value pointer comments
Merge pull request #2396 from take-cheeze/superclass_check

Move direct superclass checking to `test/t/superclass.rb`.
Merge pull request #2401 from take-cheeze/fixed_state_atexit_stack

Add fixed state atexit stack feature.
Add mrbconf documentation.
Merge pull request #2403 from take-cheeze/mrbconf_doc

Start `mrbconf` documentation.
remove superclass test for classes that do not exist in mruby (core)
update String#slice doc
Merge branch 'suzukaze-delete-string.slice_comment'
Merge branch 'master' of github.com:mruby/mruby
resolve conflict
git push origin masterMerge branch 'yui-knk-replace-assert'
fix conflict resolution mistake