Minor cleanup

[ImplOfAnImpl]

A bunch of trivial changes extracted from my other branch:

1. Partially apply an old TODO that Ids (and other 256-bit values) should be passed by ref and smaller types should be passed by value.
2. Remove redundant (and inappropriately named) function mempool::make_mempool and use MempoolInit::new directly instead.

P.S. I also had to update the keccac crate to pacify cargo deny