Documentation for MRuby::GemBox usage

add extern funcs declaration and casts for cimpiling C++ compiler

GemBox

detect cyclic-include in Module#include

Code clean-up

Avoid to define MRB_INT16 and MRB_INT64 at the same time

Remove macro ANYARGS. It is not used in the core and bundled mrbgems. And it is possible to conflict an application's macro.

Merge branch 'suppress_intern' of https://github.com/crimsonwoods/mruby into crimsonwoods-suppress_intern

Clean up load.c

Fix-up string

Add error checks for small environments which pointer is less than 32bit. These code will be removed by optimizer if the target runs on 32bit or above.