Remove old (boxed) closures from the language

[pcwalton]

We will want to remove (old) boxed closures from the language for 1.0, I believe. At the very least we should feature gate them.

Nominating for 1.0, P-backcompat-lang.

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

[Rdbaker]

How would one go about this?

[huonw]

@Rdbaker the new closures don't quite exist yet (#14539), so the best thing for now is waiting. :)

[gamazeps]

@huonw and now ? :p