Types the consumer-facing surface of ark-bls12-381, ark-ec and ark-ff so
a consumer's variables resolve through pairings and field sampling.
Both trait eras are keyed separately: ark_ec::PairingEngine (0.2-0.3)
and ark_ec::pairing::Pairing (0.4+) are different keys, not spellings of
one, so a single key would silently export "pairing(?, ?)" on half the
committed range.

Arities and parameter types were read from the trait declarations.