GC Arena docs pass for fixups

Implemented round(double) function when _MSC_VER < 1800

VisualStudio compilers where _MSC_ver < 1800

mirb should take filename and arguments.

Remove mrbtest irep

YARD doc for mrb_str_to_cstr.

add {Array|Hash|String}.try_convert

If we make the core tests a dependency on the mruby-test gem,
we don't need to worry about maintaining the core test irep alone.

Fix MRB_DISABLE_STDIO typo.

Support Numeric#zero? and Numeric#nonzero?

Fix typo on MRB_DISABLE_STDIO.

Fix check for VisualStudio Version in mruby-math gem

Bugfix nagative-number lshift() bit overflow

Update README.md

Fix mrbgems doc link.

Enable full-debug build on travis using `MRB_ENABLE_DEBUG_HOOK`

Fix MRB_ENABLE_DEBUG_HOOK config typo left from 4440566b

/cc #3014