Move managed pointers into the standard library

[bstrie]

I'm surprised that I was unable to find an open issue for this.

Read http://pcwalton.github.io/blog/2013/06/02/removing-garbage-collection-from-the-rust-language/ for the basic idea. From what I understand the general attitude is favorable, but people have yet to decide whether to salvage the @ sigil or to require @T to now be written Gc<T>.

Let's try to defer these syntax decisions and just get GC-in-a-library working as soon as possible, if people agree that it is desirable.

Nominating for 0.8. Aggressive, but I feel like we're going to suffer if we try to put this off for too long.

[bstrie]

Paging @pcwalton.

[nikomatsakis]

Paging me.

[catamorphism]

We're not really using the 0.8 milestone, but this would be a backwards-compatible milestone issue.

[catamorphism]

Accepted for milestone 1, well-defined

[pnkfelix]

cc me

[thestinger]

I think we should move managed pointers behind a feature switch ASAP. A library implementation isn't important before we actually implement a garbage collector, because the important functionality is already provided by the much better performing Rc type.

[thestinger]

The Gc<T> stub exists and implementing a garbage collector is #2997 (see #11399 too).