frost-core: split part of lib.rs into traits.rs and serialization.rs

[conradoplg]

As mentioned in #568 , lib.rs got too big so I split parts of it into traits.rs and serialization.rs.

While doing that I noticed that some types were pub but they didn't need to be, so I hid them.

[codecov]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (71c0925) 81.94% compared to head (9145519) 81.94%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #569   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files          30       31    +1     
  Lines        3151     3151           
=======================================
  Hits         2582     2582           
  Misses        569      569           

Files	Coverage Δ
frost-core/src/error.rs	84.61% <ø> (ø)
frost-core/src/identifier.rs	98.64% <ø> (ø)
frost-core/src/keys.rs	97.75% <ø> (ø)
frost-core/src/keys/dkg.rs	87.64% <ø> (ø)
frost-core/src/keys/repairable.rs	97.18% <ø> (ø)
frost-core/src/round1.rs	92.65% <ø> (ø)
frost-core/src/round2.rs	100.00% <ø> (ø)
frost-core/src/verifying_key.rs	98.52% <100.00%> (ø)
frost-ed25519/src/lib.rs	65.92% <ø> (ø)
frost-ed448/src/lib.rs	65.92% <ø> (ø)
... and 7 more

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[natalieesk]

Looks good