improve sign() performance by caching SigningCommitments

[conradoplg]

Closes #454

Benchmark:

FROST Signing ristretto255/Round 2/2
                        time:   [97.239 µs 97.456 µs 97.727 µs]
                        change: [-28.248% -28.139% -28.010%] (p = 0.00 < 0.05)
                        Performance has improved.
FROST Signing ristretto255/Round 2/7
                        time:   [180.00 µs 181.16 µs 182.72 µs]
                        change: [-19.124% -18.843% -18.525%] (p = 0.00 < 0.05)
                        Performance has improved.
FROST Signing ristretto255/Round 2/67
                        time:   [1.1946 ms 1.1955 ms 1.1966 ms]
                        change: [-3.4950% -3.2638% -3.0433%] (p = 0.00 < 0.05)
                        Performance has improved.
FROST Signing ristretto255/Round 2/667
                        time:   [11.588 ms 11.607 ms 11.629 ms]
                        change: [-1.4843% -1.2623% -1.0580%] (p = 0.00 < 0.05)
                        Performance has improved.

[codecov]

Codecov Report

Patch coverage: 93.33% and project coverage change: -0.13% ⚠️

Comparison is base (60d9942) 77.46% compared to head (eb6fa70) 77.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
- Coverage   77.46%   77.34%   -0.13%     
==========================================
  Files          30       30              
  Lines        2738     2741       +3     
==========================================
- Hits         2121     2120       -1     
- Misses        617      621       +4     

Files Changed	Coverage Δ
frost-core/src/frost/round1.rs	87.03% <92.85%> (-2.07%)	⬇️
frost-core/src/frost/round2.rs	90.90% <100.00%> (-0.30%)	⬇️

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[dconnolly]

Nice!