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.
Adds support for PHP 8 attributes, represented using `AttrGroup` nodes
containing `Attribute` nodes. The `attrGroup` subnode is added to all
nodes that can have attributes.
This is still missing FPPP support.
Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>