Implement some missing num-traits traits

All of the below obviously only applies to the types where it is applicable.

1. Most important: Num (looks like num-traits maintainers will clarify in the docs that it's ok to return Err in from_str_radix so we're good to go; as it may take them unknown amount of time to do it though, should we just go ahead and do it on our side?)
2. Also important: Signed, Unsigned (both are currently blocked by Num)
3. Inv
4. Pow (for integer power types only)
5. MulAddAssign (just for the sake of completeness)
6. FloatConst
7. ToPrimitive, FromPrimitive