The `hash_newtype` macro is explicitly designed to produce a hash that
is not a general purpose hash type to try and prevent users hashing
arbitrary stuff with it. E.g., `Txid` isn't meant to be just hash
arbitrary data. However we provide a `From` impl that will convert any
instance of the inner hash type into the new type. This kind of defeats
the purpose. We provide `from_byte_array` and `to_byte_array` to allow
folk to 'cast' from one hash type to another if they really want to and
its ugly on purpose.
Also, it is becoming apparent that we may be able to remove the `hashes`
crate from the public API of `primitives` allowing us to stabalise
`primitives` without stabalising `hashes`.
For both these reasons remove the `From` impl from the `hash_newtype`
macro. Note that deprecating doesn't seem to work so we just delete it.
6b2b89c2f7