Reduce number of reallocations

[Nashtare]

Description

Pre-allocate sufficient amounts when creating new vectors, most of the time relying on the compressed_size() method from the CanonicalSerialize trait.

Mostly useful for to_bytes methods. Some vector initializations are purposedly left untouched, either because they are within unit tests or because they deal with malicious participants which, we hope at least, wouldn't require more than 1 allocation in the worst case (i.e. at most 4 malicious participants at once).

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (7553d9a) 82.68% compared to head (9a9bf1b) 82.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   82.68%   82.55%   -0.14%     
==========================================
  Files          13       13              
  Lines        1057     1026      -31     
==========================================
- Hits          874      847      -27     
+ Misses        183      179       -4     

Files	Coverage Δ
src/dkg/complaint.rs	98.52% <100.00%> (ø)
src/dkg/key_generation.rs	91.22% <100.00%> (-0.19%)	⬇️
src/dkg/nizkpok.rs	97.22% <100.00%> (ø)
src/dkg/participant.rs	92.77% <100.00%> (-0.57%)	⬇️
src/parameters.rs	100.00% <100.00%> (ø)
src/sign/signature.rs	79.31% <100.00%> (+0.36%)	⬆️
src/sign/precomputation.rs	90.14% <66.66%> (-0.40%)	⬇️
src/dkg/secret_share.rs	79.04% <60.00%> (ø)
src/keys.rs	80.21% <71.42%> (ø)

... and 1 file with indirect coverage changes

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[hratoanina]

Random question, but if the to_bytes function is identical for a DistributedKeyGeneration<RoundTwo, _>, can't you implement both at the same time?

[Nashtare]

Yes to_bytes / from_bytes could be defined for an arbitrary DkgState, I didn't bother dealing with it because we may refactor this, following #8. But that's acknowledged, thanks for noticing 👍🏼