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/ast.rs

@@ -288,7 +288,8 @@ enum blk_check_mode { default_blk, unchecked_blk, unsafe_blk, }
 enum expr_check_mode { claimed_expr, checked_expr, }
 
 #[auto_serialize]
-type expr = {id: node_id, node: expr_, span: span};
+type expr = {id: node_id, callee_id: node_id, node: expr_, span: span};
+// Extra node ID is only used for index, assign_op, unary, binary
 
 #[auto_serialize]
 enum alt_mode { alt_check, alt_exhaustive, }