Core support for complex numbers

This is support for "fast" complex numbers in the core. Enable MRB_COMPLEX to turn it on. Normally, MRB_COMPLEX will use its own mrb_value boxing but you can also use it with MRB_WORD_BOXING.

Methods like **, abs, conj, phase, arg, and polar should probably be an mrbgem. Let me know if you agree. I will do this work after I review any changes made to this pull request.

Also, vm.c can speed up complex math in the same way as regular math. I will submit this as a separate pull request so you can decide if it should be in the core.