Reformat metadata for exported macros

Instead of copy-pasting the whole macro_rules! item from the original .rs file,
we serialize a separate name, attributes list, and body, the latter as
pretty-printed TTs.  The compilation of macro_rules! macros is decoupled
somewhat from the expansion of macros in item position.

This filters out comments, and facilitates selective imports.

src/libsyntax/fold.rs

@@ -1115,7 +1115,7 @@ pub fn noop_fold_mod<T: Folder>(Mod {inner, view_items, items}: Mod, folder: &mu
     }
 }
 
-pub fn noop_fold_crate<T: Folder>(Crate {module, attrs, config, exported_macros, span}: Crate,
+pub fn noop_fold_crate<T: Folder>(Crate {module, attrs, config, mut exported_macros, span}: Crate,
                                   folder: &mut T) -> Crate {
     let config = folder.fold_meta_items(config);
 
@@ -1146,6 +1146,10 @@ pub fn noop_fold_crate<T: Folder>(Crate {module, attrs, config, exported_macros,
         }, Vec::new(), span)
     };
 
+    for def in exported_macros.iter_mut() {
+        def.id = folder.new_id(def.id);
+    }
+
     Crate {
         module: module,
         attrs: attrs,