The UseUse::$alias node can now be null if an alias is not
explicitly given. As such "use Foo\Bar" and "use Foo\Bar as Bar"
are now represented differently.
The UseUse->getAlias() method replicates the previous semantics,
by returning "Bar" in both cases.
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.
