The literal like this `"[[table]]"_toml` caused a syntax error. It is
because the literal parser first check that it might be a bare value
without a key, and parse_array directory throws syntax_error. This
change makes the parser first check a literal is a name of table, and
then parse the content.
`location::line_num()` function used to be implemented by using
`std::count`, so each time the parser encounters a type mismatch,
`std::count` was called with almost whole file. It decelerates the
parsing process too much, so I decided to add `line_number_` member
variable to `location` and add `advance/retrace/reset` to `location`
in order to modify the position that is pointed.
