`merlin` is currently a blocker for upgrading to `rand_core` v0.9 by way
of the `transcript.build_rng().finalize()` function (which we only pass
`ZeroRng` to).
There is an open PR to update `rand_core` in `merlin` and I have pinged
the relevant people to take a look, hopefully: zkcrypto/merlin#11
However, in the event we can't get `merlin` updated, this at least
unblocks the `rand_core` upgrade, and is being opened as a contingency
plan for that case.
The PR has been implemented in a way that it should be easy to switch
back to upstream `merlin` in the event they upgrade `rand_core`.
