This introduces a new crate providing `HashReader`/`HashWriter` wrappers
implementing `AsyncRead`/`AsyncWrite`, mirroring `digest-io`'s sync API.

This only implement the tokio's async API for now.

The intent is to fold it in `digest-io` when the
`AsyncReader`/`AsyncWriter` eventually make their way into `std`.

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Co-authored-by: Artyom Pavlov <newpavlov@gmail.com>