when ""_toml literal is used with C++11 raw-string literal,
it normally starts with newline like the following.
```cpp
const auto v = u8R"(
[table]
key = "value"
)"_toml;
```
With this, the error message shows the first empty line that starts just
after `u8R"(` and thus the error message shows nothing. To avoid this,
skip the first empty lines and whitespaces in literal.
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.
