Currently we only get `std::io::Write` impls when the `bitcoin-io`
dependency is used. This is overly restrictive, it would be nice to have
`std::io::Write` imlps even without the `bitcoin-io` dependency.
Copy the logic out of the `bitcoin_io::impl_write` macro into `hashes`
but feature gate it differently.
Call the new macro inside `hash_type` (and in `hmac`), remove the
`impls` module, and move the tests to the integration test directory.
Remove the `io` feature from `hashes`, now if users enable `std` they
get `std::io::Write` impls and if they enable `bitcoin-io` they get
`bitcoin_io::Write` impls as well.
Fmt Bot
Tobin C. Harding