add version field to serialization at the beginning; move ciphersuite after it

[conradoplg]

This also adds a Header field to the structs as dicussed.

This also changes the Ciphersuite::ID to be the contextString from the spec, per @dconnolly 's suggestion. My arguments in favor of that are:

• It's something concrete from the spec, rather then the old value which does not have a specific name
• It includes a version. If the ciphersuite in the spec changes for whatever reason and contextString changes, then it's basically a new ciphersuite and any serialized FROST structs for that ciphersuite will be incompatible

I didn't change Ciphersuite::ID to be Ciphersuite::CONTEXT_STRING because it doesn't seem to gain us much, but I'm still undecided, feedback is welcome.

Closes #534

[github-actions]

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.12% 🎉

Comparison is base (eee505a) 81.17% compared to head (81f4ac1) 81.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
+ Coverage   81.17%   81.29%   +0.12%     
==========================================
  Files          30       30              
  Lines        3012     3032      +20     
==========================================
+ Hits         2445     2465      +20     
  Misses        567      567              

Files Changed	Coverage Δ
frost-ed25519/src/lib.rs	63.56% <ø> (ø)
frost-ed448/src/lib.rs	63.56% <ø> (ø)
frost-p256/src/lib.rs	67.80% <ø> (ø)
frost-ristretto255/src/lib.rs	63.77% <ø> (ø)
frost-secp256k1/src/lib.rs	67.58% <ø> (ø)
frost-core/src/frost.rs	92.36% <100.00%> (ø)
frost-core/src/frost/keys.rs	98.42% <100.00%> (ø)
frost-core/src/frost/keys/dkg.rs	89.13% <100.00%> (ø)
frost-core/src/frost/keys/repairable.rs	97.14% <100.00%> (ø)
frost-core/src/frost/round1.rs	92.65% <100.00%> (ø)
... and 2 more

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[natalieesk]

Just a few typos but otherwise looks good.