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).
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`.
