Add conversion functions between `bls12_381_fr` and `nat`/`int`.

This is a followup issue to !367 (merged) closing this discussion.

In !367 (merged), the INT instruction is extended to work from bls12_381_fr to int but no direct instruction is provided to go from bls12_381_fr to nat nor from nat or int to bls12_381_fr (!367 (merged) provides simple but indirect ways to do these).

Do we want these extra instruction?