Implement MiniscriptKey for each variant of DescriptorPublicKey

[Velnbur]

The same as rust-bitcoin/rust-miniscript#938 but for elements

[apoelstra]

utACK -- heads up we may need to first port ElementsProject/rust-elements#266 and ElementsProject/rust-elements#269 to get our tooling up to date here. (Though tentatively it looks like things are working.)

[apoelstra]

I'm also failing to pass the elementsd-tests tests on my local CI box. I am getting

---- test_arith stdout ----

thread 'test_arith' (50) panicked at tests/setup/mod.rs:76:48:
called `Result::unwrap()` on an `Err` value: JsonRpc(Transport(SocketError(Os { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" })))
stack backtrace:

With elementsd 23. IIRC there is some fairly simple change we need to make to fix our tests for a more recent elements.

[apoelstra]

ACK 2a852f2; successfully ran local tests