Add workaround for math functions not defined in BigInteger

We add support for:

 - conjugate
 - magnitude
 - log
 - angle
 - atan
 - acos
 - asin
 - tan
 - cos
 - sin

by converting to a FloatPoint and computing the results there.

We also define round() as simply returning the given integer itself because
every BigInteger is an integer already.