for the new token parsingcode path to use that one unified implementation too.
The previous implementation dispatched like parse() -> prefix_parse(), the
latter of which is incompatible with token parsing.
parser_interface. token_spec is now a variable template that generates a
parser_interface wrapping a token_parser, which parameterized on the
token_spec_t. This way, a single token_spec use can be used to specify how to
lex, and how to parse.
See #202.
