Internal functions can only be called from within the library.
Functions listed in `mruby/internal.h` can be called from:
* core (src/*.c)
* gems (mrbgems/**/*.c)
But not from the application linked with `libmruby`.
To more consistent names.
* mrb_num_plus() -> mrb_num_add()
* mrb_num_minus() -> mrb_num_sub()
Since no one seems to use those functions, there should be no problem.
We added migration macros for compatibility for safety.