Add a map method to Variables

[mdiep]

As a way of adding constraints

// should find "mouse", "moose"
let fourLetterWords: AnyIterator<String> = solve { word in
    return word.map { $0.length } == 5
        && any(
            word == "dog",
            word == "cat",
            word == "mouse",
            word == "moose",
        )
}

[mdiep]

Done in bacd8ac5e172d3c798b76cec129e1e89215b5788.