Fix error handling for mrb_generate_code().

The return value of mrb_generate_code()
must be null-checked before referencing.

Fix error handlings in mirb.

mrb_parser_new() and mrb_generate_code() may return NULL.

refactor mruby method(fix indent. remove temporary value etc)

Improve replacement math functions for Visual C++.

Refactor classc

removed unused variable in range_beg_len

Fix indents in numeric.c

Refactor flo_mod func in numeric.c

Remove some roundoff from mrb_flo_to_str

refactor mrb_hash_shift, num_pow, flo_eq, flo_finite_p method

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.