Do signature recovery/verification with 4 possible recid case
Merge pull request #122

6e05287 Do signature recovery/verification with 4 possible recid case (Pieter Wuille)
Require that r and b are different for field multiplication.

Suggested by Peter Dettman, this prepares for slightly faster muitiplication
which writes results immediately to r before finishing reading b.
Make secp256k1_fe_mul_inner use the r != property

Suggested by Peter Dettman.