Currently we require indexing trait bounds as well as `Borrow` on the
`Hash` trait. We also already implement `AsRef`.
It was observed that `Borrow<[u8]>` does not best describe what we want
from the `Hash` trait implementor but rather `AsRef<[u8]>` does.
Remove all the inexing trait bounds. Remove the `borrow::Borrow<[u8]>`
trait bound. Add a `convert::AsRef<[u8]>` trait bound.
This leaves the `Borrow<[u8]>` implementation for hashes created with
`hash_newtype`, I'm not sure if this should be removed or not.
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