Consider impl MiniscriptKey for secp256k1::PublicKey

[dr-orlovsky]

secp256k1::PublicKey is the only type that can be used to derive both XOnlyPublicKey and bitcoin::PublicKey. This pattern is useful a lot in downstream wallets, when they do not know which key type will be held by descriptors.

[apoelstra]

Yeah this seems like a good idea.

Our decision not to use secp256k1::PublicKey predates the Miniscript::is_uncompressed method, which now can simply be set to constant false for secp key types.

[dr-orlovsky]

@apoelstra did that in #327