Reduce new string creation in `capitalize`, `downcase`, `upcase` method of `Symbol` class extension.
Add test to check unmodified result is working.

Proc class should be defined before a first method definition

Update documentation of `summary` property of mrbgem spec.

Fix document of  doc/compile/README.md

Fix document in doc/mrbgems/README.md

Fix document of doc/mrbconf/README.md

Syntax highlight code blocks with github flavored markdown.

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.

mrb_define_method setting mrb->proc_class for method proc