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.
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.
[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.