Add `Descriptor::desc_type` method

[thunderbiscuit]

Proof of Concept. Will keep iterating. The parsing of some miniscripts that are according to the BIP valid also fails, and I don't know exactly why but assume it's because those are not supported by the miniscript crate? Not sure yet (you can see some of the tests I commented out).

Descriptors taken from the BIPs here: https://github.com/bitcoin/bips/blob/master/bip-0380.mediawiki

See #687.