[plt-scheme] contract constructors

I think the following contract constructors would be useful. I'm not sure
if they'd be hard/impossible.

    ;; I imagine (promiseof a) should be similar to (-> a)
    (promiseof contract)                                   ; ho

    ;; similar to vector/p
    (struct/p struct-type flat-contract ...)               ; flat

    ;; similar to immutable-vectorof
    (immutable-structof struct-type contract ...)          ; ho

Dave