Add a new crate `bitcoin-units`, move the `amount` module over to it and re-export all types from `bitcoin::amount` so this as not a breaking change.
AFAICT we literally never used this; it was available only on the bitcoin targets and not the honggfuzz ones; AFL has a broken dep tree (or at least, requires some more MSRV pins that I did not care to investigate). Just remove it entirely.