refactor: make Descriptor plan API available for generic Pk

[trevarj]

• Migrate plan and plan_mall methods from DefiniteDescriptorKey to generic
  Pk: MiniscriptKey + ToPublicKey.

• Made the Plan struct generic over Pk (excluding update_psbt_input since
  it needs descriptor key data).

Adding this to same PR in a separate commit since it's trivial:

• Rename plan and plan_mall to have into_ prefix

Closes #927 and #928

[apoelstra]

In fe71f3e:

Can you re-add the old name as a deprecated passthrough to the new name?

[trevarj]

In fe71f3e:

Can you re-add the old name as a deprecated passthrough to the new name?

Done. Wasn't sure which version to use in since. Left it as 13.0.0 since I don't know what the next release will be (13.1.0?)

[tcharding]

In rust-bitcoin we throw 'TBD' in there and then try to set it before release. Not sure what we want to do here though?

[apoelstra]

Either TBD or 14.0.0 would be fine.

If you're really motivated you can use 13.1.0 but then you need to make a separate PR to backport this deprecation to the 13.x branch. But the next release off master will be the 14.x series.

[trevarj]

Pushed with "TBD" there is another one floating in the code too, so we can change all 3 to 14.0.0 when the time comes.

[apoelstra]

CI is failing because there's a non-whitelisted call to the deprecated method somewhere.

[trevarj]

CI is failing because there's a non-whitelisted call to the deprecated method somewhere.

Right...rebased onto master. There was a usage of path in a662453

[trevarj]

@apoelstra I think CI needs to be re-run:

Run cargo install --git https://github.com/rust-bitcoin/rust-bitcoin-maintainer-tools.git --rev "$(cat rbmt-version)" cargo-rbmt
    Updating git repository `[https://github.com/rust-bitcoin/rust-bitcoin-maintainer-tools.git`](https://github.com/rust-bitcoin/rust-bitcoin-maintainer-tools.git%60)
warning: spurious network error (3 tries remaining): unexpected http status code: 500; class=Http (34)
warning: spurious network error (2 tries remaining): unexpected http status code: 500; class=Http (34)
warning: spurious network error (1 try remaining): unexpected http status code: 500; class=Http (34)
error: failed to clone into: /home/runner/.cargo/git/db/rust-bitcoin-maintainer-tools-027d3079154a3847

Caused by:
  revision bcff6290c9e3f5b4d7e832acb5a8c6bf890fd1ce not found

Caused by:
  network failure seems to have happened
  if a proxy or similar is necessary `net.git-fetch-with-cli` may help here
  https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli

Caused by:
  unexpected http status code: 500; class=Http (34)
  

[apoelstra]

ACK b646120; successfully ran local tests