fixed evaluation context of eval(string) and instance_eval(string)

removed non-need space

Fix typo in gc.c

Rename GC_STATE_NONE GC_STATE_ROOT

Refactor incremental_sweep_phase() in gc.c

Fix a typo in etc.c

mruby-strip doesn't need mrbgems

Refactor obj_free() in gc.c

revert accidental constify.

added Math.cos and Math.tan test case

fix linkage of range_beg_len and mrb_range_beg_len

Reduces executable size by more than 50% (409->171 KB).

MSVC: add simple (v)snprintf implementation

change linkage of mrb_str_size to internal

fix conversion warnings (in gc.c)

remove unnecessary CHAR_BIT != 8 check

Use RSTRING_LEN to get the length.

_snprintf is not C99's snprintf!

This simple implementation, unlike _snprintf,
always null-terminates and returns the expected
return value (in most cases).
Other C99 behaviors (like format specifiers) require
adding a complete snprintf implementation.
Do we want or need that?

The same applies to vsnprintf (aka _vsnprintf).

Those warnings are not enabled by default,
but getting rid of them doesn't hurt.