The trait is implemented by default for u8 and u64, allowing pow_vartime
to be used with both the byte encoding and limb representation of field
elements.
WARNING: THIS IS NOT ACTUALLY CONSTANT TIME YET!
The jubjub and bls12_381 crates will replace our constant-time usages,
but we NEED to fix ff_derive because other users will expect it to
implement the Field trait correctly.
The ff_derive, pairing, zcash_primitives::jubjub, and bellman dummy_engine
changes are minimally implemented on top of the existing *_assign()
functions.
