Rewrite each_path to allow performance improvements in the future.

Instead of determining paths from the path tag, we iterate through
modules' children recursively in the metadata. This will allow for
lazy external module resolution.

src/libstd/unstable/extfmt.rs

@@ -94,6 +94,7 @@ use iterator::IteratorUtil;
 #[doc(hidden)]
 pub mod ct {
     use char;
+    use container::Container;
     use prelude::*;
     use str;