This fixes the long-standing issue where a comment would get assigned
to all nodes with the same starting position, instead of only the
outer-most one.
Fixes#253.
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.
I'm somewhat unsure about the AST structure here.
VariadicPlaceholder is not a general expression. Maybe Arg->expr
should be Expr|VariadicPlaceholder? Or possibly the call arguments
should be an array of Arg|VariadicPlaceholder?
