Move RacyCell to std::comm

RacyCell is not exactly what we'd like as a final implementation for
this. Therefore, we're moving it under `std::comm` and also making it
private.

src/libstd/thread.rs

@@ -127,7 +127,7 @@
 use any::Any;
 use borrow::IntoCow;
 use boxed::Box;
-use cell::RacyCell;
+use comm::RacyCell;
 use clone::Clone;
 use kinds::{Send, Sync};
 use ops::{Drop, FnOnce};