* nested list()s will now create nested List nodes (instead of just
nested arrays)
* yield $k => $v was parsed with key and value swapped. This is now fixed
* the pretty printer now works with the newly added language constructs
Example: foreach ($coords as list($x, $y)) { ... }
This change slightly breaks backwards compatability, as it changes the
node structure for the previously existing `list(...) = $foo` assignments.
Those no longer have a dedicated `AssignList` node; instead they are
parsed as a normal `Assign` node with a `List` as `var`. Similarly the
use in `foreach` will generate a `List` for `valueVar`.
