Note [FamInstEnv determinism]

We turn FamInstEnvs into a list in some places that don't directly affect
the ABI. That happens in family consistency checks and when producing output
for `:info`. Unfortunately that nondeterminism is nonlocal and it's hard
to tell what it affects without following a chain of functions. It's also
easy to accidentally make that nondeterminism affect the ABI. Furthermore
the envs should be relatively small, so it should be free to use deterministic
maps here. Testing with nofib and validate detected no difference between
UniqFM and UniqDFM.
See Note [Deterministic UniqFM].