Undid some of the modifications...

- Copied digit.h and cuda_string.h back to mpz/ (I need them
in order to compile and test...)

- Added digit.h and cuda_string.h back into the Makefile (they
will definitely change)

- Left the include/ dir but I don't think we need it since we
are working orthogonally right now. We can figure out our project
layout when we start merging and writing the real code.
Implemented multiplication (mpz_mult())

Algorithm was simply long multiplication. It has been tested and
seems to work well.

I'll probably try to karatsuba next.