The sqrt() function is now part of the Field trait. ff_derive returns an
error on fields for which it does not support generating a square root
function.
Note that Fq6 and Fq12 in pairing::bls12_381 leave the function
unimplemented. They will be dropped once the migration to the bls12_381
crate is complete. The equivalent structs in that crate are not exposed.
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.
