feat: add .set_sign() and Mul<Sign>/MulAssign<Sign> for BigInt

[virchau13]

An attempt at solving #226. Adds .set_sign() and Mul<Sign>/MulAssign<Sign> for BigInt.

The current behaviour is that .set_sign(s) sets the sign if s is either Plus or Minus. If s is NoSign, the BigInt is set to zero. Mul<Sign>/MulAssign<Sign> is equivalent to self.set_sign(self.sign() * self.rhs), using the already-present impl Mul<Sign> for Sign.