Remove GMP field implementation
Merge pull request #168

7277fd7 Remove GMP field implementation (Pieter Wuille)
Make signing fail if a too small buffer is passed.

Bug discovered by Sergio Demian Lerner.
Merge pull request #169

603c33b Make signing fail if a too small buffer is passed. (Pieter Wuille)
weak normalization
Weak normalization for secp256k1_fe_equal
Add _fe_normalizes_to_zero method
Add _normalizes_to_zero_var variant