Add explicit notion of copyability for closures.

[bblum]

Proposal: A closure is copyable iff everything in its environment is copyable. Furthermore, non-copyable ("one-shot") closures are allowed to capture and de-initialise outer variables.

This will allow building closures that run destructors for you. It will also fix #2828.

Closures' types can then be written with copy and send traits. fn x(f: fn()) { f(); f(); } will no longer be legal; you have to write fn x(f: fn()<copy>) or some-such.

[bblum]

This will also have problems with the new proposed task builder interface (and also, if i'm not mistaken, with the old one too!).

This will fail:

let t = task::task().notify_port(p);
t.spawn(body);
t.spawn(body);

This will not fail but should:

let t = task::task().notify_port(p);
for iter::repeat(100) { t.spawn(body) }

[nikomatsakis]

~fn() closures are becoming non-copyable, making this a non-issue.

[bblum]

Yeah, as long as #2549 stays open, this doesn't need to be open.