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.
so far, the error value of the lexer is just ignored because they are
not readable (results from all the nested combinator are concatenated,
so they are too redundant). those ones are replaced by a simple literal.
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.
