Jez and Andy write a Parser

My friend Jez is back, and we make a start on a parser for the Cell language (in Rust, of course).

You can find Jez at jezuk.co.uk and follow him at @jezhiggins@mastodon.me.uk.

Soon after we finished recording, Jez reported to me that he'd got it passing all tests, and provided this code snippet:

fn next_node(&mut self, prev: Option<AST>) -> Option<AST> {
    match self.next_token() {
        Some(token) => self.build_node(token, prev),
        None => prev.and(make_ast_error("Hit end of file - expected ';'.")),
    }
}