Add Scalar.SetUInt64, remove Scalar.SetBigInt, add Hex() and DecodeHe…

[bytemare]

…x(), add tests

[codecov]

Codecov Report

Attention: Patch coverage is 95.78947% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.82%. Comparing base (b866f41) to head (fc74736).
Report is 16 commits behind head on main.

Files	Patch %	Lines
internal/edwards25519/scalar.go	86.66%	1 Missing and 1 partial ⚠️
internal/ristretto/scalar.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   81.47%   90.82%   +9.34%     
==========================================
  Files          16       19       +3     
  Lines         934     1439     +505     
==========================================
+ Hits          761     1307     +546     
+ Misses        112       76      -36     
+ Partials       61       56       -5     

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[sonarcloud]

Quality Gate failed

Failed conditions
16.7% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud