This adds an additional "returnType" subnode to Stmt\Function_,
Stmt\ClassMethod and Expr\Closure, as well as the corresponding
support in the name resolver and pretty printer.
* 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