Pre multi-keychain work (do not merge)

[thunderbiscuit]

Opening this following the dev call discussion from August 4th.

This PR is not meant for merging. Its purpose is to separate the work on the multi-keychain feature (#227) into the parts that:

1. Are required for the feature but not directly related to it (work we expect to land on master before the multi-keychain feature and simplify the multi-keychain PR). This primarily includes the removal of the signers. Other things could come here (for example if the decision was made to remove the TxBuilder entirely, that would come into this part), but the goal of this segment is to not get bogged down into those details and just propose some rough draft of what might be, or order to enable part 2:
2. The actual PR that proposes a new API shape. In theory, this makes it easier to keep the review and discussion on that PR on the exact feature, and not carry the noise that part 1 brings in (changes of tests from tx_builder to create_psbt, or rewiring of examples because signers don't exist in the same way anymore, etc.)

Notes to the reviewers

Take a look at this if you want. But the interesting part of the multi-keychain feature are in #524.

I will try to keep this rebased on master as well as incorporating my best understanding of what is likely to land for 4.0 if things change. If you have comments on this please let me know! I'll try to keep this PR up to date with the best expected direction.

TODO

• Make sure we rebase this on Deprecate wallet-owned signing APIs #505 and keep the changes there landing here
• Orthogonal but I don't want to forget about it: LoadParams::descriptor -> LoadParams::check_descriptor

[codecov]

Codecov Report

❌ Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.79%. Comparing base (fc88144) to head (9ad7073).

Files with missing lines	Patch %	Lines
src/wallet/mod.rs	92.30%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   81.84%   81.79%   -0.05%     
==========================================
  Files          25       25              
  Lines        6487     6362     -125     
  Branches      296      289       -7     
==========================================
- Hits         5309     5204     -105     
+ Misses       1078     1058      -20     
  Partials      100      100              

Flag	Coverage Δ
rust	81.79% <94.33%> (-0.05%)	⬇️

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