A useful closure + fn pointer optimization

[cdstanford]

(moving this a new thread so we can tag it roadmap-v1, c.f. #34)

Just discussed this with David, false positives for closure creation seem to come up commonly, e.g. for closures like |x| x + 1 which are quite common.

An extremely useful optimization we should do here is:

• if the closure contains no effects inside it -- so absolutely no effects are possible when called -- skip adding the ClosureCreation effect.

[cdstanford]

done in 9f45ed7, thanks @lzoghbi !