Implement a key resharing protocol

[fjarri]

A part of #20. Not closing it yet since I'm not certain it's the best protocol to use, and it needs to be properly integrated with the rest of the framework. Also there is one pathway where a possible error is not attributable to any party - can be avoided by assuming that each party has access to public parts of keyshares of other nodes.

Merging this now because it has grown pretty big. Still more work needed to close the mentioned issues.

Public:

• Added a "key resharing" protocol based on T. M. Wong, C. Wang, J. M. Wing "Verifiable Secret Redistribution for Archive Systems"(https://www.cs.cmu.edu/~wing/publications/Wong-Winga02.pdf, https://doi.org/10.1109/SISW.2002.1183515). Specifically, REDIST protocol.

Internal:

• Switched from using HoleVec to BTreeMap when accumulating messages from other nodes.
• Added a Finalizable trait to handle tasks or checking whether the round can be finalized, and if not, whose messages are missing. (see Which layer should implement the check of whether a round can be finalized? #85)
• Added num_parties() and party_idx() methods to Round
• Added ShareIdx type (a part of Support one party owning several key shares #31, decoupling PartyIdx from the share index)

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[codecov]

Codecov Report

Attention: 62 lines in your changes are missing coverage. Please review.

Comparison is base (01c94b0) 86.48% compared to head (2cafdfc) 87.18%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
synedrion/src/cggmp21/protocols/key_resharing.rs	88.30%	40 Missing ⚠️
synedrion/src/cggmp21/protocols/threshold.rs	89.61%	8 Missing ⚠️
synedrion/src/sessions/states.rs	80.00%	8 Missing ⚠️
synedrion/src/sessions/type_erased.rs	94.54%	3 Missing ⚠️
synedrion/src/cggmp21/protocols/generic.rs	97.75%	2 Missing ⚠️
synedrion/src/cggmp21/protocols/test_utils.rs	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   86.48%   87.18%   +0.70%     
==========================================
  Files          44       45       +1     
  Lines        6644     7251     +607     
==========================================
+ Hits         5746     6322     +576     
- Misses        898      929      +31     

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