Import the _random module

Hmm do we also need to do some two's-complement conversion here?  ex if bytes is 0xffff, it looks like the string we'll generate is "-ffff", which won't come out to -1.  I'm not sure if GMP has good support for this; maybe one option is that if we detect it's signed (is_signed && sign_bit_is_set), we just subtract 1<<(8*n) at the end.

or we could just assert that `is_signed == 0` :P

Also, it looks like mpz_import() could be useful here.
https://gmplib.org/manual/Integer-Import-and-Export.html