Open up Target protocols

[gemedet]

This PR updates the protocol requirements for ConstraintConstantTarget, ConstraintOffsetTarget, and ConstraintInsetTarget to be public (just like ConstraintMultiplierTarget), allowing for conformance by custom types:

enum Margin: Int, ConstraintOffsetTarget {
    case small = 10
    case large = 20
}

...

make.left.equalToSuperview().offset(Margin.small)

I also included some RawRepresentable extensions, so that opting-in can be as easy as putting it in the type declaration. If that's out of scope, these can be omitted, leaving implementations to the user.

[Overcot]

Maybe rebase to new develop?