Re-export digest crate

[mqudsi]

In order to interact with meowhash::MeowHasher functions implementing
digest::Digest (required, for example, to calculate the hash of a
stream without buffering it in-memory entirely), it's necessary for
downstream users to use digest::Digest to gain access to the impl.

There are two problems:

1. meowhash users need to add two dependencies to their Cargo.toml
   file in order to use this crate (a dependency on meowhash and a
   dependency on digest), and
2. It's insufficient to just (also) add a dependency on digest, you
   have to find out what version meowhash is using internally and
   match that version when declaring the dependency, otherwise two
   versions of the digest crate end up being used: one being the
   version meowhash depends on, and the other being the digest
   version pulled in by the updated Cargo.toml. They're incompatible,
   so putting use digest::Digest in your code and then trying to use
   <MeowHasher as Digest>::update(...) causes rustc to emit errors
   because MeowHasher doesn't actually implement the same
   digest::Digest that one is using.
3. Even if you did match your digest dependency against that used by
   meowhash, if you ever cargo upgrade there's a very good chance
   everything will break again.

The solution for this in the rust ecosystem is to pub use your
external dependency. You could probably get away with just pub use digest::Digest instead of pub use digest since the latest version of
meowhash doesn't expose any digest API members other than the
Digest trait (generic_array is no longer exposed), but just
re-exporting the entire crate seems to be the preferred approach in the
crate ecosystem and it's less of a maintenance burden down the line (in
case you do end up using something else from the digest crate in your
public API once again).