The change to the pinned circuit description is due to removing the
unnecessary `selector_map` pin in zcash/halo2#521. This invalidates
previous proofs due to changing Fiat-Shamir transcript initialization).
We are about to extract the sub-configs from mul::Config and refactor
them. Doing so would have moved their gate definitions past the one gate
that isn't created in a sub-config. Reordering the definitions here will
make the subsequent refactor diffs simpler to review.