Now that we are able to unambiguously go from a hash engine to its
associated hash type there is no longer any need for the `GeneralHash`
trait.
Please note that IMO this concept of a general hash type as opposed to
one where one can hash arbitrary data still exists in the codebase - it
is implicitly in the `hash_newtype` macro.
Remove the `GeneralHash` trait.
95ad91cdb6