[racket] Dave Herman's memoize package

Jens Soegaard's message was the second time in two days that I spent more than thirty seconds grumbling about the fact that the memoize.plt planet package depends on the schemeunit planet package and hence on a whole bunch of stuff, so I decided to do something about it.

In fact, it turns out that Dave had already done the work himself! ... but not released it. So, in the spirit of "act now, apologize if necessary," I forked his GitHub repo and released it as a package to pkg.racket-lang.org. The code is all still Copyright 2012 David Herman.

Anyhow, that's done: you can stop relying on planet for memoize.

Hope this is helpful for some folks,

John