fix non-human-readable serde deserialization

[apoelstra]

No description provided.

[apoelstra]

IMHO this requires only a minor version bump since it strictly increases the set of things that can be deserialized.

Prior to this PR we would attempt to deserialize hashes first as &[u8], which we would copy into the hash type. The Deserialize impl for &[u8] uses a Visitor object that can only handle "borrowed bytes" (which are slices with a sufficiently long lifetime that they can be returned to callers; compare "bytes", which are also borrowed, but which are not assumed to outlive the invocation of Visitor::visit_bytes).

We don't need our byteslices to live long enough to to be returned, since we just copy the bytes into a new Hash object and return that. So we implement our own visitor which does exactly the same thing as before, but implements visit_bytes rather than visit_borrowed_bytes (and the default impl of visit_borrowed_bytes will fallthrough to visit_bytes).

This is needed to roundtrip using serde_cbor and probably other binary deserializiers.