clean up the Rc type and move it to libstd

[thestinger]

I've left out a way to construct from a Send type until #9509 is resolved. I am confident that this interface can remain backwards compatible though, assuming we name the Pointer trait method borrow.

When there is a way to convert from Send (from_send), a future RAII-based Mut type can be used with this to implemented a mutable reference-counted pointer. For now, I've left around the RcMut type but it may drastically change or be removed.

[bill-myers]

Note that this line is wrong, both in the existing Rust codebase and in this commit:
let _: ~T = transmute(self.ptr);

it should be:
let _: ~RcBox = transmute(self.ptr);

since it is and is created as an ~RcBox and not a ~T

[thestinger]

@bill-myers: nice catch! fixed

[thestinger]

@bill-myers: interestingly enough, because value is the first field in the struct, the mistake never actually caused any issues - but it should be fixed regardless, because that's way too fragile.