Simplify _fastmath (remove import)

The randfunc parameter of getRandomInteger() can
be a callable object or None.

If it is None, C code will import the Random module
and create a new RNG object.

Apart from breaking pypy, the C code is ugly and unnecessary:
it is much easier to ensure that getRandomInteger()
is always called with a valid object from the Python side.
Make AESNI.c more C89-compliant.
Stop poking inside PyLong's

This patch modifies the 2 routines to convert
libgmp integers into (and from) PyLongs, so
that internals or PyLongs are not accessed
directly anymore.

With this last patch, PyCrypto should run under PyPy.