Julia scoping

[arnavs]

Not an issue currently since the examples we chose work under both "soft" and "hard" scoping rules. But the previous example on the website (i.e., the standard i = 0; while i < 10; i += 1; end) doesn't. Applies only to the pure-Julia cheatsheet.

cc @jlperla (and connected to what you said about better iterators). This could be another reason to replace the pure-Julia sheet with the one from Julia Computing.