Add temporal contracts

[disnet]

Add support for temporal contracts. The idea is described in this paper and a Racket implementation is here.

There are two parts. One is the guard/monitor interface and the second is the language/automaton.

We'll have to think about how the syntax fits into CoffeeScript but it should be something like this describing a sort function that is not re-entrant (can only be called after all previous calls to sort have returned) and cannot call cmp after sort has returned:

sort :: ([Pos], cmp : (Pos, Pos) -> Bool) -> [Pos]
  where not ... call(sort, _) !ret(sort, _)*
                call(sort, _)
    and not ... ret(sort, _) ... call(cmp, _)
sort = (l, cmp) -> ...

Note that cmp : (Pos... is doing name binding and is not supposed to be an object contract. Will need to figure out an unambiguous naming syntax.