fn expect_ident(&mut self) -> Result<IdentId, ParseErr> {
        match &self.get()?.kind {
            TokenKind::Ident(s) => Ok(self.get_ident_id(s)),
            _ => Err(error_unexpected(self.prev_loc(), "Expect identifier.")),
        }
    }
Get the next token and examine whether it is Ident.
Return IdentId of the Ident.
If not, return RubyError.
ruruby-parse/src/parser.rs