extend ChangeHash internal initializer to use Automerge core logic

[heckj]

From #191 :

In the automerge crate we have a TryFrom<&[u8]> implementation for ChangeHash. This is the supported way of converting bytes into a ChangeHash and is where we would handle future inconsistencies in hash format (not that I expect any at this point). If possible I think it would be best to expose this method up through the Uniffi UDL rather than assuming that any 32 byte array is a valid change hash. (This also applies to the serialization step, except we're already using ChangeHash::as_bytes when we generate the bytes which we pass to the Swift side).