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/libsyntax/lib.rs

@@ -36,6 +36,7 @@ extern crate collections;
 #[phase(syntax, link)]
 extern crate log;
 extern crate fmt_macros;
+extern crate debug;
 
 pub mod util {
     pub mod interner;