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.
This makes a basic effort and has not been audited.
Doesn't appear to have a measurable performance impact on bench.
It also adds a secp256k1_num_free to secp256k1_ecdsa_pubkey_create.
- Uses a similar approach to the latest 64bit _normalize.
- Add one useful optimization back into the 64bit _normalize too.
Performance of 'bench' improved by around 0.5% for the 32bit field (but tested on a 64-bit machine).
