Rewrite boxed_region/memory_region in Rust

This drops more of the old C++ runtime to rather be written in rust. A few
features were lost along the way, but hopefully not too many. The main loss is
that there are no longer backtraces associated with allocations (rust doesn't
have a way of acquiring those just yet). Other than that though, I believe that
the rest of the debugging utilities made their way over into rust.

Closes #8704

src/libstd/unstable/raw.rs

@@ -15,8 +15,8 @@ use unstable::intrinsics::TyDesc;
 pub struct Box<T> {
     ref_count: uint,
     type_desc: *TyDesc,
-    priv prev: *Box<T>,
-    next: *Box<T>,
+    prev: *mut Box<T>,
+    next: *mut Box<T>,
     data: T
 }