- change value_t::typename from CamelCase to snake_case.
- drop CamelCase typename supports.
The changes are introduced to make the interfaces uniform. For some
(historical) reasons, toml11 has both CamelCase names and snake_case
names for types. Additionally, since `float` is a keyword, snake_case
names uses `floating` to avoid collision and CamelCase name uses `Float`
because toml official calls it `Float`. This is too confusing.
Since it is a major upgrade, I think it is a big chance to make them
uniform.
Although the error value from combinators currently does not have any
information, it can have an information because it is a char value. It
is better to use no-information-type explicitly to make it clear that
it does not have any information. So I added none_t in toml::detai and
use it in combinators and parsers as an error value from combinators.
Generate error message in `parse_something()`, not in `lex_something`.
Since the error message generated by `lex_something` is too difficult to
read for humans, I've disabled the error message generation for the sake
of efficiency (it takes time to generate error message that will never
be read). I think now the error message generation itself safely can be
removed from combinators. At this stage, `lex_something` does not need
to return `result<T, E>` because all the error type would be discarded.
Now it is turned out that returing `optional<T>` from lex_* is enough.
Maybe later I would change the return type itself, but currently I
changed the error type from std::string to char because implementing
optional takes time and effort. It makes the parsing process a bit
faster.
