Demode some code using by-mutbl-ref; warn about by-mutbl-ref

The parser now warns about use of mutbl-ref mode, though it's kind
of a lie since this commit doesn't remove support for the mode.

Changed move_val_init to have stage0 and stage1/2 versions, the latter of
which is demoded.

Changed the type that the typechecker expects the move_val_init
intrinsic to have. After this is pushed, I can make a new snapshot,
which will remove the need for the stage0 versions.

src/libsyntax/parse/parser.rs

@@ -570,6 +570,7 @@ impl parser {
 
     fn parse_arg_mode() -> mode {
         if self.eat(token::BINOP(token::AND)) {
+            self.warn(~"Obsolete syntax has no effect");
             expl(by_mutbl_ref)
         } else if self.eat(token::BINOP(token::MINUS)) {
             expl(by_move)