Adding this as an option to avoid breaking people's tests.
Some of the test results show pretty clearly that we are incorrectly
assigning the same comment multiple times for nested nodes (mentioned
in #36).
* 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`.
