Derive Hash for SpendableOutputDescriptor

[TheBlueMatt]

This allows for easy descriptor de-duplication before building a claiming transaction.

[TheBlueMatt]

Trivial and user-requested, tagging 117.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (1ac53ed) 88.84% compared to head (8a42d55) 88.85%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2602   +/-   ##
=======================================
  Coverage   88.84%   88.85%           
=======================================
  Files         113      113           
  Lines       84729    84729           
  Branches    84729    84729           
=======================================
+ Hits        75275    75282    +7     
- Misses       7240     7241    +1     
+ Partials     2214     2206    -8     

Files	Coverage Δ
lightning/src/sign/mod.rs	74.40% <0.00%> (ø)

... and 11 files with indirect coverage changes

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[tlulu]

Worth updating the docs to indicate Hash can be used as unique identifier?

[TheBlueMatt]

I think its implied, at least if you're using a hasher which is sufficiently strong (the default one is siphash returning a 64-bit int, so it should be fine...). In any case, the usual Rust hash API applies.