Refactor tuple pattern edge-case in genBinding

[nojaf]

This refactors genBinding to move the actual workaround we have for tuples.

Concretely, we have this for:

        let var1withAVeryLongLongLongLongLongLongName, var2withAVeryLongLongLongLongLongLongName = // foo
            someFunc 1, someFunc 2

we need to wrap (var1, var2) into parentheses to remain valid code.

But we were repeating a bit too much code in genBinding to deal with this.

[dawedawe]

nice!