Add support for testing quadratic residues #402

1. 
   sipa commented at 11:39 am on July 4, 2016: contributor
2. Add Jacobi symbol test via GMP

   Also add native Jacobi symbol test (Andrew)
   
   Rebased-by: Andrew Poelstra
   Rebased-by: Pieter Wuille

   efd953a7a7
3. Add function for testing quadratic residue field/group elements. e6e9805fc4
4. sipa force-pushed on Jul 4, 2016
5. 
   apoelstra commented at 11:56 am on July 4, 2016: contributor

   ACK
6. sipa merged this on Jul 4, 2016
7. sipa closed this on Jul 4, 2016

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8. sipa referenced this in commit b3401237be on Jul 4, 2016
9. gmaxwell cross-referenced this on Aug 27, 2020 from issue Add schnorrsig module which implements BIP-340 compliant signatures by jonasnick

Contributors
sipa apoelstra