Inline filter after a join

[hadley]

@mgirlich could you please take a look at this? Probably easiest to see exactly what's happening in 94e3206; the other commit is just some refactoring to make the changes a bit easier.

Fixes #722

[hadley]

Hmmmm, this is now correct, but I don't love the implementation. It's inconsistent to undo the aliasing in add_select() for select queries but in sql_build() for joins. The problem is that we don't appear to have a helper to join all the pieces together earlier, partly because generating the join names seems to need the connection. I wonder if this is reflecting that we really need some sort of table-variable nesting system here so that you could write df$x and have it translated to "DF"."x".