turn ast::ident into a struct

This will help with the auto_serialize2 migration. We have to change
ident from a type alias to uint into a unique type. We need to use
a struct instead of a "enum ident = token::str_num" because structs
support constants, but newtypes do not.

src/libsyntax/ast_util.rs

@@ -23,7 +23,7 @@ pure fn dummy_sp() -> span { return mk_sp(0u, 0u); }
 
 
 
-pure fn path_name_i(idents: ~[ident], intr: token::ident_interner) -> ~str {
+pure fn path_name_i(idents: ~[ident], intr: @token::ident_interner) -> ~str {
     // FIXME: Bad copies (#2543 -- same for everything else that says "bad")
     str::connect(idents.map(|i| *intr.get(*i)), ~"::")
 }