Nearly all special errors are now handled gracefully, i.e. the
parser will be able to continue after encountering them. In some
cases the associated error range has been improved using the new
end attribute stack.
To achieve this the error handling code has been moved out of the
node constructors and into special methods in the parser.
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
