[plt-dev] abstract contracts

I've added #:exists to provide/contract, as a way to hide information
ala "type t" declarations on ML signatures.

See the contracts section in the Guide for a worked example and a
discussion of a gotcha. The short version is that you can now write
things like this:

(provide/contract
  #:exists stack
  [new stack]
  [push (-> int stack stack)]
  [pop (-> (and/c stack non-empty?) int)]
  [non-empty? (-> stack boolean?)])

and have the contract system enforce data abstraction, even if your
stack operations are simply these:

  (define new '())
  (define push cons)
  (define pop car)
  (define non-empty? pair?)

That is, clients of your module will not be able to treat your stacks
as if they were lists, even though they really are lists.

Robby