changed some string method's mrb-aspec

mrb_str_bytesize is a carbon copy of mrb_str_size.  remove it.

mruby core does not support any character encodings but ASCII.

fix build when neither src nor mrblib directory exists.

Fix String#succ. "-".succ should be "."

changed mrb_str_size and mrb_str_bytesize comments.

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