Move std::{reflect,repr,Poly} to a libdebug crate

This commit moves reflection (as well as the {:?} format modifier) to a new
libdebug crate, all of which is marked experimental.

This is a breaking change because it now requires the debug crate to be
explicitly linked if the :? format qualifier is used. This means that any code
using this feature will have to add `extern crate debug;` to the top of the
crate. Any code relying on reflection will also need to do this.

Closes #12019

[breaking-change]

src/liballoc/lib.rs

@@ -74,8 +74,10 @@
 extern crate core;
 extern crate libc;
 
+
 // Allow testing this library
 
+#[cfg(test)] extern crate debug;
 #[cfg(test)] extern crate sync;
 #[cfg(test)] extern crate native;
 #[cfg(test)] #[phase(syntax, link)] extern crate std;