fix: allow parsing Descriptor<DescriptorPublicKey> from pk with xprv

[trevarj]

Refactor and fix FromStr for DescriptorPublicKey so that it can parse a
xprv/tprv by doing the conversion into the DescriptorPublicKey.

Added test that reflects what was reported in the issues:
bitcoinfuzz/bitcoinfuzz#70
#785

[apoelstra]

This feels a little dangerous to me. If the user provides secret data and then we irreversibly throw it away, could that cause a loss of the secret?

[trevarj]

we irreversibly throw it away

Yeah, that's a good point. Using parse_descriptor seems like the best route since you get back the KeyMap.
Wonder if FromStr should just be removed. I feel like most people will see parse_descriptor in the API before they consider calling parse(), or maybe that's just me.