Define and use ##bignum.copy, simplify integer division, have gcd use le...

...ss memory.

Marc:

Now bignum division generates two cons cells (basically) if you don't need the quotient and you allow the dividend to be overwritten.  This is helpful for small bignum gcd, but it still generates two cons cells for each 1.5 bits (roughly) in the arguments.  A binary algorithm would probably be faster in this regime.