Add support for setting to columns in addition to parameters in UPDATEs

@brianc: Currently, it's not possible to do an `UPDATE table1 SET table1.col1 = table2.col2 ...` because updates assume a `ParameterNode()`.

I've made it work, but I'm not sure about my implementation because there is probably other code relying on `this.value` being a raw value instead of a `Node`. I couldn't think of another way off the top of my head that wouldn't have a significant impact on the API... any ideas?