SHA512_rng() asserts GMP_NUMB_BITS==64

[tromer]

SHA512_rng() in rng.tcc contains this assert:

assert(GMP_NUMB_BITS == 64); // current Python code cannot handle larger values, so testing here for some assumptions.

This breaks some of the executables on 32-bit platforms (e.g., the src/zk_proof_systems/pcd/r1cs_pcd/r1cs_mp_ppzkpcd/tests/test_r1cs_mp_ppzkpcd test).

Can the assert be relaxed to <= 64?

Also, let's clarify the comment -- which Python code is it talking about, when we're deep in C++ land?