Provide unmarshalling and marshalling functions for data types

[torkelrogstad]

It would be a good idea to provide implementations of some common encoding formats for data structures in btcutil. Especially json.Marshaller and json.Unmarshaller I can imagine being useful for users that build applications or services that interact with Bitcoin data structures over JSON-based APIs.

Is there any opinions out there on this? I'm happy to do some implementation work.

[Roasbeef]

Transferred from the old btcutil repo.