Implement GetKey for KeyMap

[tcharding]

Create a KeyMap type to replace the current BTreeMap alias and implement bitcoin::psbt::GetKey for it.

Close: #709

[tcharding]

cc @LLFourn

[apoelstra]

Nice! Yeah, this is probably the right approach.

In 00342a1 you have a bunch of unreachables because you assume that public keys map consistently to secret keys. I think this is a reasonable assumption, but you need to enforce it in the insert method. Or perhaps you could change insert to just take a secret key, and do the pubkey computation itself.

Alternately, you could make insert be non-pub so that the type can only be constructed by calling Descriptor::parse_descriptor. This might be the better approach. (Though in this case I'd still add an assertion to insert checking consistency of the public and secret keys.)i

[LLFourn]

Thanks @tcharding. It might be nice to be able to append your key maps onto each other so you keep one big jumble of secret keys across all your descriptors in the application. This is possible now but would be harder to do if you make the field private (which does seem like the right thing in general). So maybe an .extend with another keymap. If you make it private you probably want to make a way to iterate it too.

[tcharding]

Yes I threw the pub inner field so as not to have to implement the whole BTreeMap API but private inner is probably better as you say - I can do this now we a have loose concept ack.

[ValuedMammal]

Concept ACK

[sanket1729]

It might be nice to be able to append your key maps onto each other so you keep one big jumble of secret keys across all your descriptors in the application.

Maybe in the longer term, a cleaner solution is to implement rust-bitcoin to implement GetKey for a tuple, vec, set of things that already implement GetKey.

impl_get_key_tuple!((T1, T2))
impl GetKey for Vec<T> where T: GetKey {
}
impl GetKey for BtreeSet<T>...

[sanket1729]

Concept ACK.