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/parse/parser.rs

@@ -237,7 +237,7 @@ struct parser {
     mut restriction: restriction,
     mut quote_depth: uint, // not (yet) related to the quasiquoter
     reader: reader,
-    interner: interner<@~str>,
+    interner: @token::ident_interner,
     keywords: HashMap<~str, ()>,
     strict_keywords: HashMap<~str, ()>,
     reserved_keywords: HashMap<~str, ()>,