toLowS = function(s) {
  if (s.gt(new BN('7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0', 'hex'))) {
    s = Point.getN().sub(s);
  }
  return s;
}
Convert s to a low s
see BIP 62, "low S values in signatures"
@param {BN} s
@returns {BN}