78ec6fe30c
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