Add Elm.Constraint.checkConstraint

[MartinSStewart]

I wanted to write an elm-review rule that would verify that an Elm package has a valid Elm constraint. For example, 0.19.1<=v<0.20.0 is valid and 0.0.0<=v<1.0.0 is not. Unfortunately, it's not easy to verify this without Elm.Constraint.checkConstraint: Constraint -> Constraint -> Bool. That is to say, a function that takes two constraints and checks that the first one is contained in the second one.

An alternative approach would be to implement #5 since this checkConstraint can be written by the user if Version and Op are exposed.