Function jtm::brainpool_p256r1::ecdsa_sign

pub fn ecdsa_sign<F>(z: BigUint, d: BigUint, f: F) -> (BigUint, BigUint) where
    F: FnMut(BigUint) -> BigUint, 

f(BigUint) -> BigUint is supposed to be a random number generator. Return a number from 0 to "less than" $1. In rare cases it'll be called multiple times, it's imperative that a diffrent number is returned!