Add min/max functions to Constraint

[malaire]

For package dependency calculations I need to get the min/max value of Constraint.

So I suggest adding two new functions which return min/max Version:s and their associated Op:s:

min : Constraint -> (Version, Op)
max : Constraint -> (Version, Op)

Note that this requires exposing Op(..).

(I removed my earlier idea here, this is better and can also have other uses with those exposes Op:s, e.g. with custom conversion to string or something else.)