Allow assignment right member to reference columns

Such queries become possible for mortals:

```ruby
table.where(table[:qux].eq 'zomg').compile_update(table[:foo] => table[:bar])
```

Arguably much more sane than using `engine.connection.quote_column_name`
or `engine.connection.visitor.accept` on the right hand (bar), which is
totally leaking the abstraction.