Don't highlight multiple identifiers if not allowed

Keywords like insteadof, instanceof and "as" are followed by exactly one
identifier, so only that identifier is now highlighted. An exception to
that rule is "extends"; java-mode too allows a list of classifiers there
even though it doesn't support multiple inheritance.
Highlight class name resolution constant

PHP 5.6 introduces the ::class constant (e.g. stdClass::class) so it
makes sense to highlight it with a constant face