Update gearley to cfg's latest version

[trickster]

There are a lot of changes to cfg crate, it's a good API now. This crate however, I could update it to latest version with some changes but the grammar.rs is very difficult to migrate

For example

fn populate_grammar_with_events_rhs(&mut self, grammar: &BinarizedGrammar) {
        self.events_rhs[1].extend(
            grammar.rules().map(|rule| rule.history().dot(1).event_without_tracing())

What's it's equivalent to the new cfg?