The parser will now always generate Identifier nodes (for
non-namespaced identifiers). This obsoletes the useIdentifierNodes
parser option.
Node constructors still accepts strings and will implicitly create
an Identifier wrapper. Identifier implement __toString(), so that
outside of strict-mode many things continue to work without changes.
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.
