Use E1/E2::ScalarField instead of Fp/Fq

[dannywillems]

Using Fq/Fp required to add more type constraints when working on #2938. Therefore, I suggest to always use E1/E2::ScalarField.

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.84%. Comparing base (8bb7b7a) to head (a88fc63).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
arrabbiata/src/witness.rs	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2981      +/-   ##
==========================================
- Coverage   76.85%   76.84%   -0.02%     
==========================================
  Files         261      261              
  Lines       61370    61370              
==========================================
- Hits        47168    47161       -7     
- Misses      14202    14209       +7     

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