In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this
representation measurably improves memory usage and performance.
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.
