add a few const qualifier

Fix to not print unnecessary comma.

Add `Symbol#intern`.

Start documenting C APIs.

Add mrbconf document.

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