use max_signers and min_signers instead of num_signers and threshold to better follow spec

[conradoplg]

Depends on #156 because it touches the same code.

The current naming can be confusing because in the spec, num_participants is the number of participants creating a signature, while in the code it's the maximum number of participants.

This renames them to become more similar to the spec. However, I decided to keep "signer" instead of "participant" because we use that terminology in a bazillion other places. We can make the change in a separate PR if we decide to.

This is a cleanup without a matching issue.

[codecov-commenter]

Codecov Report

Base: 83.61% // Head: 86.45% // Increases project coverage by +2.83% 🎉

Coverage data is based on head (b8170cb) compared to base (bea4ef0).
Patch coverage: 93.90% of modified lines in pull request are covered.

❗ Current head b8170cb differs from pull request most recent head f1af814. Consider uploading reports for the commit f1af814 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
+ Coverage   83.61%   86.45%   +2.83%     
==========================================
  Files          17       18       +1     
  Lines        1398     1779     +381     
==========================================
+ Hits         1169     1538     +369     
- Misses        229      241      +12     

Impacted Files	Coverage Δ
frost-core/src/frost/round2.rs	70.42% <ø> (ø)
frost-core/src/lib.rs	72.97% <0.00%> (-6.44%)	⬇️
frost-p256/src/lib.rs	74.34% <75.00%> (+2.19%)	⬆️
frost-ristretto255/src/lib.rs	74.83% <76.47%> (+2.37%)	⬆️
frost-core/src/frost/identifier.rs	75.47% <78.04%> (+1.05%)	⬆️
frost-core/src/frost/keys.rs	80.88% <95.40%> (+1.11%)	⬆️
frost-core/src/frost/keys/dkg.rs	96.55% <96.55%> (ø)
frost-core/src/tests.rs	99.55% <99.31%> (-0.45%)	⬇️
frost-core/src/frost.rs	91.81% <100.00%> (-0.28%)	⬇️
frost-core/src/frost/round1.rs	90.96% <100.00%> (+0.11%)	⬆️
... and 3 more

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[dconnolly]

max and min, does what it says on the tin