Obliterate the callee_id hack

Exprs that could be applications of overloaded operators
(expr_unary, expr_binary, expr_index) relied on the previous node ID
being "reserved" to carry extra typechecking info. This was
incredibly error-prone. Fixed it; now all exprs have two node IDs
(which will be wasted in some cases; future work could make this
an option instead if the extra int field ends up being a performance
problem).

Closes #2804

src/libsyntax/fold.rs

@@ -688,6 +688,7 @@ impl of ast_fold for ast_fold_precursor {
     fn fold_expr(&&x: @expr) -> @expr {
         let (n, s) = self.fold_expr(x.node, x.span, self as ast_fold);
         ret @{id: self.new_id(x.id),
+              callee_id: self.new_id(x.callee_id),
               node: n,
               span: self.new_span(s)};
     }