Now we have an associated const we can do away with the `engine` trait
method all together. Users can call `Hash<FooTag>::engine` instead. This
is better because its an API more similar to the other hash types and
therefor easier to discover and remember.
Instead of requiring users of the `Tag` trait to implement the `engine`
method we can have an associated const and provide an `engine` method
with a default implementation.
Use an associated const for the pre-tagged hash engine. Fro now keep the
`engine` trait method but have a default impl that returns the const. We
will remove it as a separate patch to assist review.
This macro is a maintenance burden. We would like to put a tag on the
hash engine but doing so would require breaking this macro anyway so
lets just delete it.
In preparation for removing a bunch of macros move all the modules to
`<mod>/mod.rs`.
Do so by running the following shell:
```bash
for mod in hash160 ripemd160 sha1 sha256 sha256d sha256t \
sha384 sha512 sha512_256 siphash24 hkdf hmac; do
mkdir $mod
mv "$mod.rs" "$mod/mod.rs"
done
```
Internal change only.
Tobin C. Harding