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.
Instead of storing subnodes in a subNodes dictionary, they are
now stored as simple properties. This requires declarating the
properties, assigning them in the constructor, overriding
the getSubNodeNames() method and passing NULL to the first argument
of the NodeAbstract constructor.
[Deprecated: It's still possible to use the old mode of operation
for custom nodes by passing an array of subnodes to the constructor.]
The only behavior difference this should cause is that getSubNodeNames()
will always return the original subnode names and skip any additional
properties that were dynamically added. E.g. this means that the
"namespacedName" node added by the NameResolver visitor is not treated
as a subnode, but as a dynamic property instead.
This change improves performance and memory usage.
