Add context to signature-related errors

[djc]

Fixes #356.

This could use some feedback. In particular, the macro encoding of alg_tests is fairly painful to update for this. Any suggestions how best to do that? I started on unwrapping some the macro test wrappers in another branch but that would a decent amount of boilerplate (though it does make things a bit more obvious).

[codecov]

Codecov Report

Attention: Patch coverage is 97.65258% with 5 lines in your changes missing coverage. Please review.

Project coverage is 97.98%. Comparing base (8081620) to head (085ee9b).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/crl/mod.rs	0.00%	4 Missing ⚠️
src/error.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
- Coverage   97.99%   97.98%   -0.02%     
==========================================
  Files          20       20              
  Lines        4729     4901     +172     
==========================================
+ Hits         4634     4802     +168     
- Misses         95       99       +4     

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[djc]

Moved the algorithm encodings from src/data/test to tests/signatures/alg-* -- because this is test-only, seems better to keep them outside src.