Add pict/code/eval with typeset-interaction, a la scribble/eval

[lexi-lambda]

This adds a pict/code/eval module that provides interaction and typeset-interaction, which allows typesetting REPL interactions as picts, much like interaction from scribble/eval.

The way I've implemented it, this introduces a new dependency on scribble-lib to take advantage of make-base-eval. If this is problematic or if it should be avoided, it would be possible to implement it another way, instead (though it would likely involve implementing something very similar to make-base-eval, anyway).

[mflatt]

I would like to avoid a dependency of pict-lib on scribble-lib. Should make-base-eval move to its own package? Or is it thin enough but useful enough to add (with a better name) to racket/sandbox?

[samth]

I don't particularly care about this case, but in general I think we should aim for more, smaller, packages rather fewer larger ones. This approach has worked very well in a number of other language communities, and I think it would be good in Racket as well.

[lexi-lambda]

I'd be fine with moving it into racket/sandbox or moving it into its own package. Do you have a particular preference?