Fix interaction between magnitudes and negation

Magnitude m means values are allowed to be up to 2 * 0xFFF...FFF * m,
while the argument passed to secp256k1_fe_negate didn't take the 2 into
account. Fix this.
Merge pull request #85

7a8e385 Fix interaction between magnitudes and negation (Pieter Wuille)
warnings: enable quiet builds
Merge pull request #93

5362875 warnings: enable quiet builds (Cory Fields)
Address 'constant-time' TODOs in field impls
Merge pull request #92

137e77a Address 'constant-time' TODOs in field impls (Peter Dettman)
Fix typo
Label variable-time functions correctly and don't use those in sign
Merge pull request #94

da55986 Label variable-time functions correctly and don't use those in sign (Pieter Wuille)
Implement Schnorr signatures