Allow user to provide their own digest algorithm, remove sha3 dependency

[dralley]

Let user use whichever implementation of digest::Digest they like.

closes #20

[dralley]

Since the verify feature would no longer require a heavy dependency (digest::Digest is just a trait and the digest crate doesn't have many deps) maybe it would make sense to have verify be provided by default?

[hunger]

Could you do a quick cargo fmt? Or shall I?

I use pre-commit to make sure to not forget to clean up when committing:-)

[dralley]

It's an API break yes. Because the old verification function (downloader::verify::sha3_256) is gone

[dralley]

I ran cargo fmt

[hunger]

Thanks! Let's see what the Ci says now:-)