Split `Liveness::users` into three.
Split `Liveness::users` into three.

This reduces memory usage on some benchmarks because no space is wasted
for padding. For a `check-clean` build of `keccak` it reduces `max-rss`
by 20%.

aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust