Midstates are not generic objects; they don't have universal
cryptographic properties and if you are using them you should be using a
specific midstate type. Therefore it shouldn't be part of `GeneralHash` or
`HashEngine`. Furthermore, in practice it seems like `sha2` midstates are
the only ones that anybody uses, at least in bitcoin.
Remove the midstate stuff from the `GeneralHash` and `HashEngine`
traits. Keep the `midstate` functionality as inherent functions if it is
used internally. Keep the functionality on `sha256` as inherent public
functions.