Rounding infinite Rationals

[sostock]

Rounding 1//0 and -1//0 only works when a rounding mode is given:

julia> round(1//0, RoundNearest)
1//0

julia> round(1//0)
ERROR: DivideError: integer division error

I will make a PR for this later.

Edit: trunc, floor and ceil also do not support infinite Rationals.