Support Output Descriptors

[zoop-btc]

I would propose to create a library for handling output descriptors similar to what btcutil/hdkeychain does.
It is a standard which a lot of wallets follow and it would be useful to be able to import such wallets or export them in this format.

[Roasbeef]

That would be super cool. I think a portion of the implementation may end up living within btcwallet as well: https://github.com/btcsuite/btcwallet