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.
to avoid unncessary error message generation, check the first some
characters before parsing it. It makes parsing process faster and
is also helpful to generate more accurate error messages.
all the parsers generate error messages and error message generation is
not a lightweight task. It concatenates a lot of strings, it formats
many values, etc. To avoid useless error-message generation, first check
which prefix is used and then parse special integers. Additionally, by
checking that, the quality of the error message can be improved (later).
At the earlier stage of the development, I thought that it is useful if
lexer-combinators generate error messages, because by doing this,
parser would not need to generate an error message. But now it turned
out that to show an appropriate error message, parser need to generate
according to the context. And almost all the messages from lexer are
discarded. So I added another parameter to lexer-combinator to suppress
error message generation. In the future, we may want to remove messages
completely from lexers, but currently I will keep it. Removing those
unused message generation makes the parsing process faster.
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.
the following code was okay in the last release
```
toml::format_error("[test]", v, "test", {"hint1", "hint2"})
```
but was not okay in the current master. This commit fixes this.
cons: By this, the number of values to show is limited upto 3.
