Add secp256k1_fe_verify in field_5x52_impl and use it everywhere #44

1. 
   sipa commented at 5:10 pm on July 15, 2014: contributor

   Adapt @peterdettman’s verify function (taking the magnitude leniency factor 2 into account), and use it as consistency check everywhere.
2. 
   sipa commented at 1:53 pm on July 16, 2014: contributor

   @peterdettman Any comments?
3. Test demonstrating discrepancy in sqr output 59447da38d
4. Correct secp256k1_fe_verify and use it everywhere 21f81a8469
5. 
   sipa commented at 7:08 pm on July 17, 2014: contributor

   Rebased.
6. sipa merged this on Jul 23, 2014
7. sipa closed this on Jul 23, 2014

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8. sipa referenced this in commit 7d1956ff8d on Jul 23, 2014
9. sipa cross-referenced this on Sep 2, 2014 from issue Test demonstrating discrepancy in sqr output by peterdettman

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