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