add MRB_INT_BIT
add mrb_strlen_lit which makes _lit macros safer

strlen(3) + string literal is usually optimized but strlen(3) doesn't
check if its argument is really a string literal. This is important for
mruby's _static functions to which some _lit macros are expanded (string
literals have static storage). See comment for some additional info.

remove unnecessary parentheses
add including limits.h.

Because Android's libc(bionic) defines SIZE_MAX at limits.h.
Merge pull request #1850 from cremno/safer-lit-macros

add mrb_strlen_lit
Merge pull request #1851 from tmash06/add_include_limit_h

add including limits.h.
Merge pull request #1849 from cremno/add-mrb_int_bit-macro

add MRB_INT_BIT
add mrb_open_without_mrbgems API
make default allocf public to use it with mrb_open_without_mrbgems