Consider adding `control` dependency and `Alternate` instance

[hdgarrood]

See discussion in purescript/purescript-prelude#179

[JordanMartinez]

To clarify, is this just adding Alternate as a newtype here and adding an Invariant instance for it?

[hdgarrood]

Not quite - Alternate now lives in control, see purescript/purescript-control#50. The idea is to add a control dependency on this library so that we can add an Invariant Alternate instance here.