Parsable::parse_fn must be removed

[vaivaswatha]

The only reason it exists is because we need a ParserFn to be stored in Dialect::ops, and I couldn't figure out a way to store a pointer to a function that takes no argument and returns Box<dyn Parser<StateStream<'a>, Output = Self::Parsed, PartialState = ()> + 'a> . So ParserFn takes a dummy &() argument, and hence Parsable::parser_fn exists.