Remove dependency on nightly rust

[archshift]

Specifically:

• UnsafeCell can be used to emulate !Sync
• Vec may be used instead of manually using buffer + capacity + allocated_size (and depending on liballoc)

[polyfractal]

++ PR's welcome if you have the time! It'd be great if SPSC worked on stable I'm unfortunately going to be pretty busy for the rest of this month (moving houses).

I suppose the only concern with using Vec is that it just passes the buck: we'd depend on libcollections instead of liballoc, which is a slightly larger barrier for minimal embedded systems. But then again we depend on std directly so I guess that is a bigger problem for embedded. :)

[dotellie]

I really don't want to be rude, but is there any chance of this happening any time soon?