feat: implement GetKey for KeyMap and DescriptorSecretKey

[oleonardolima]

fixes #709

Description

I've built upon Tobin's work in #765 (see #765 (comment)) to implement the GetKey trait for KeyMap, a feature that will enable us to entirely remove the signers API from bdk_wallet, see: bitcoindevkit/bdk_wallet#70 and bitcoindevkit/bdk_wallet#235.

Notes to the reviewers

I followed some of Poelstra's previous comments to reduce the number of required matching branches, and also some VM's suggestions on implementing the main GetKey logic for DescriptorSecretKey instead.

Let me know if you have any other suggestions or comments.

Changelog notice

# Added:
 - implemented `GetKey` trait for `KeyMap`
 - implemented `GetKey` trait for `DescriptorSecretKey`

# Changed
 - moved `KeyMap` to it's own `key_map` module, replacing the `BTreeMap` alias.

[tcharding]

Mad, thanks man. I'm off travelling this week but can help review this when I get a chance if needed. I don't do many reviews in miniscript in general.

[apoelstra]

In 255a61e:

You need to update Cargo-minimal.lock and Cargo-recent.lock to require bitcoin 0.32.6 since the KeyRequest enum was smaller in 0.32.5 and before.

Relatedly, your unreachable! branch is not actually unreachable. It will be hit by any additions to the KeyRequest enum in the future.

[oleonardolima]

In 255a61e:

You need to update Cargo-minimal.lock and Cargo-recent.lock to require bitcoin 0.32.6 since the KeyRequest enum was smaller in 0.32.5 and before.

Relatedly, your unreachable! branch is not actually unreachable. It will be hit by any additions to the KeyRequest enum in the future.

Thanks, I've added a new commit 8e05d85 specifically for this version bump.

I also updated unreachable! to unimplemented! instead.

[apoelstra]

I also updated unreachable! to unimplemented! instead.

This is not sufficient. The code should not panic on key accesses.

[ValuedMammal]

Is this the path derived from the master key or from the tpriv shown above?