I totally have no idea when std::count returns a negative value, but the
result type of `std::count` is a differnce_type. So when it is added
with size_t value, implicit sign conversion happens. This changes check
this kind of (almost trivial but required) checking.
table = {key = "value"} # comment.
a value named "table" ({key = "value"}) has the above comment.
but a value named "key" ("value") does not have any comment.
in some cases, `region` contains several lines and `region::size`
returns the whole size that is a sum of lengthes of all the lines.
To avoid too long underlines, restrict the length of underline by
the length of the line that is shown in the message.
`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.
location is almost same as an Iterator, but having shared_ptr that points
the content. region is almost same as a range. by adding pointer to the
content source, utility function to show the error message can be
implemented easier. it is expected that this also makes easy to show
error messages after parse (e.g., in the case of bad_get)
