Make libwally values easier to debug

[Sjors]

Perhaps this should be done upstream in libwally-swift.

Is it possible to produce more useful previews of things like HDKey and fingerprints for debugging?

[w-i-n-s]

@Sjors We just need to implement protocol CustomStringConvertible in the right way. In many cases we use .description as a value, not just a description for debugging.