BIP-388 wallet policies for descriptors support

[trevarj]

Hi friends,

Creating this issue to track or discuss
BIP-388
support in this crate.

I have a draft PR
up, which is my take on how I thought it should be implemented. It is
non-invasive and hardly touches any other modules. It's a draft because of the
chance that I am way off on the implementation or missed something major!

Feel free to close this and the PR if supporting this BIP isn't in the scope of
the crate. Otherwise, I can un-draft the PR and we can start iterating over it.

Thanks!

[apoelstra]

Yep, concept ACK having BIP 388 support in this crate.