perf: cache current line number in location

`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.

toml/literal.hpp

@@ -39,7 +39,7 @@ inline ::toml::value operator""_toml(const char* str, std::size_t len)
     // literal is a TOML file (i.e. multiline table).
     if(auto data = ::toml::detail::parse_toml_file(loc))
     {
-        loc.iter() = loc.begin(); // rollback to the top of the literal
+        loc.reset(loc.begin()); // rollback to the top of the literal
         return ::toml::value(std::move(data.unwrap()),
                 ::toml::detail::region<std::vector<char>>(std::move(loc)));
     }