add a NonManaged kind

[thestinger]

This is needed to implemented weak pointers for reference counted pointers without adding explicit support in the compiler. They unable to allocate/deallocate with ~T because the object and allocation may have to be separately freed.

A dead simple implementation: https://github.com/thestinger/rust-core/blob/master/core/weak.rs

[thestinger]

It's unclear if this will be necessary. It's a problem for a garbage collector implementation to deal with.