Calling `wpubkey_hash` on a key that is uncompressed is flat out an
error, really it is a programmer error at build time because a segwit
key should never be compressed, however, for historical reasons we do
not enforce this in the type system. As a step towards clarity make it
an error to call `wpubkey_hash` on a an uncompressed pubkey. This adds
documentation and potentially might assist debugging for newer devs.
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