In codeworld-base, deprecate exp, replace with e

I'm not sure if I want to do this or not... but it occurs to me that "exp" as a function is an artifact of the computer programming world, and that mathematicians are quite happy to just write e^x. So let's do that.

Disadvantage: it steals a one-letter name. However, no one ought to be defining one-letter top-level names anyway, and using `e` as a local variable name would still be possible (and give a warning).

No middle school student is ever likely to use either one, so this isn't a big deal. Just trying to be more faithful to the mathematics world.