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