Overload the static form to reduce verbosity

Static pointers are rarely used naked: most often they are defined at the base of a Closure, as defined in e.g. the distributed-closure and distributed-static packages. So a typical usage pattern is:

    distributeMap (closure (static (\x -> x * 2)))

which is more verbose than it needs to be. Ideally we'd just have to write

    distributeMap (static (\x -> x * 2))

and let the static pointer be lifted to a Closure implicitly. i.e. what we want is to overload static literals, just like we already overload list literals and string literals.

This could be achieved by introducing the IsStatic type class and changing the typing rule for static forms slightly:

  static (e :: t) :: IsStatic p => p t