Signed improvements

[fjarri]

• Split Signed and Bounded into SecretSigned, PublicSigned, and SecretUnsigned.
• More careful handling of secret values in SecretSigned/Unsigned (fixes Zeroizing follow-up: Signed #165, fixes Wrap totient_nonzero in a SecretBox  #146)
• Simplify Exponentiable trait (part of Multiplication/exponentiation speed-ups #34)
• Simplify ring-Pedersen methods (kind of Group similar methods using traits #60)

Remaining work (for other PRs):

• Fix Zeroize MontyParams of RSA primes #162 (have to wait until the next release of crypto-bigint)
• Make Scalar always-secret too? Do we ever use non-secret scalars?

[codecov]

Codecov Report

Attention: Patch coverage is 97.43809% with 30 lines in your changes missing coverage. Please review.

Project coverage is 93.26%. Comparing base (3a45cdd) to head (f817f12).

Files with missing lines	Patch %	Lines
synedrion/src/uint/secret_signed.rs	96.69%	14 Missing ⚠️
synedrion/src/uint/public_signed.rs	93.33%	8 Missing ⚠️
synedrion/src/cggmp21/interactive_signing.rs	76.00%	6 Missing ⚠️
synedrion/src/paillier/encryption.rs	99.00%	1 Missing ⚠️
synedrion/src/uint/secret_unsigned.rs	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   93.16%   93.26%   +0.09%     
==========================================
  Files          34       36       +2     
  Lines        7026     6858     -168     
==========================================
- Hits         6546     6396     -150     
+ Misses        480      462      -18     

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[dvdplm]

Pheew, a big one!

Overall LGTM; left some nitpicks, suggestions and questions.

There's some inconsistency in what is documented and what isn't, and I think we should avoid asserts in favour of Result wherever possible, although I don't consider it a blocker for this PR.

[dvdplm]

I think this is good to go yeah?

[fjarri]

I was waiting for your response to the last round of changes, and I'm guessing you're fine with them. Merging.