Should `PublicKey`'s `Debug` show the compact form?

[TheBlueMatt]

PublicKey's Display shows the compact form that is commonly used in bitcoin, but Debug does not. At least in lightning-land this is pretty annoying cause trying to Debug structs gives a useless public key encoding that doesn't match with anything else. ISTM Debug would be more useful with the compact pubkeys.

[apoelstra]

This has been the behavior since #745

Probably we should backport it. The previous behavior was worse than "uncompressed representation"; it was actually a dump of the internal libsecp representation, which is missing a version byte and has no guarantees about stability.