[Crypto] BLST pairing

[tarakby]

• introduce Fp12 to represent pairing e target group G_T elements.
• add Fp12 comparison to the multiplicative identity element
• implement multi_pairing function that takes two arrays of G_1 and G_2 elements and computes the product in G_T ; e(G_1[0], G_2[0]) * .. * e(G_1[n], G_2[n]) using only BLST tools (batched miller loop, final exponentiation)
• Use the new multi_pairing for all BLS-related verifications in the package, and remove Relic pairing:
  • simple verification of one message and one key.
  • verifications of many couples (key, messages) batched per key
  • verifications of many couples (key, messages) batched per message
  • SPoCK verification
• clean up many unused Relic functions after the changes.

[codecov-commenter]

Codecov Report

Merging #4377 (b6b90d6) into feature/blst-based-crypto (eae8458) will increase coverage by 32.29%.
The diff coverage is n/a.

@@                      Coverage Diff                       @@
##           feature/blst-based-crypto    #4377       +/-   ##
==============================================================
+ Coverage                      18.99%   51.29%   +32.29%     
==============================================================
  Files                             52      688      +636     
  Lines                           4858    61583    +56725     
==============================================================
+ Hits                             923    31587    +30664     
- Misses                          3857    27476    +23619     
- Partials                          78     2520     +2442     

Flag	Coverage Δ
unittests	51.29% <ø> (+32.29%)	⬆️

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