Handling of special cases in EC arithmetic.
Removed test for point equality in ecdsa_verify_digest, point_add() already handles that.
Account for case when point.y == 0 when doubling.
Keep results after point_add() and point_double() inside the finite field. Simplified point_is_negative_of().
Removed superfluous bn_mod, it's done now in point_add and point_double.
Merge pull request #7 from hiviah/ecdsa_arithmetic_fix

Handling of special cases in ECDSA point arithmetic