Previously, users of TokenSyntax would always deal with RC<TokenSyntax>
which is a subclass of RawSyntax. Instead, provide TokenSyntax as a
fully-realized Syntax node, that will always exist as a leaf in the
Syntax tree.
This hides the implementation detail of RawSyntax and SyntaxData
completely from clients of libSyntax, and paves the way for future
generation of Syntax nodes.
