use as_something() instead of it. To realize this, the implementation of
as_something() is also changed. Now as_something does not depends on
`cast`. This reduces complexity around casting toml::value to other types.
Actually, since `floating` is used for toml::types, `as_floating`
seems to be clearer. But currently `is_*` functions uses `float`,
not `floating`, so `as_float` is chosen for the consistency.
In a future release, possibly v3, those names may need to be
re-considered for clarity.
- comment_before(): get comments just before a value.
- comment_inline(): get a comment in the same line as a value.
- comment(): get comment_before() + comment_inline().
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.
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).
