Note [InstEnv determinism]

We turn InstEnvs into a list in some places that don't directly affect
the ABI. That happens when we create 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 also Note [Deterministic UniqFM]