don't always generate gem functions

timer on windows bug fixed.

Fix a bug that class variable can't be referenced from class method

Class method defined in singleton class should be evaluated in class
context not singleton class context.

fix #2515

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).

If the src and mrblib directories of a mrbgem don't exist or don't
include "usable" files (for tools like mirb), then functions for gem
initialization and finalization don't have to be generated.

MSVC: add simple (v)snprintf implementation