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.

The issue concerns the codeworld-base names exp and e, but it does not identify files, tests, or an entry point. First locate where these names are defined and referenced, then determine the compatibility and warning behavior required; done means the intended replacement and deprecation behavior are implemented consistently.

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