[racket] Typed Racket interface to plot

Working from Ray Racine's initial Typed Racket interface to `plot', I've 
added the module `plot/typed'. It should be in the next nightly build, 
if something I did earlier for it doesn't continue to break the build. 
It shouldn't. :D

Example use:

#lang typed/racket

(require math plot/typed)

(plot3d (contour-intervals3d
          (λ: ([x : Real] [y : Real])
            (/ (gamma x) (gamma y)))
          0.25 2 0.25 2)
         #:angle 250
         #:out-file "gamma-ratio.pdf")


Every documented `plot' and `plot/utils' export is now typed, and they 
pass the new `plot/typed' tests. There are probably small errors remaining.

Two things to note:

  * For Typed Racket users: non-keyword optional arguments are currently
    required. We're waiting on a fix in TR for this. It shouldn't take
    long. In the meantime, including all non-keyword arguments doesn't
    seem to be a serious imposition, because they're usually required to
    get sensible plot bounds. (For multiple plots, use `#f'.)

  * For all plot users: functions that used to accept lists and vectors
    now accept arbitrary sequences and sequences of sequences. Even the
    function passed to `vector-field' can return a sequence.

Ray: thanks a ton for doing most of it. It took me just over a day 
instead of a week because of your initial work.

Neil ⊥