Support std::optional members for TOML11_DEFINE_CONVERSION_NON_INTRUSIVE

[ken-matsui]

This PR enables support for std::optional members for TOML11_DEFINE_CONVERSION_NON_INTRUSIVE. If a user-defined struct has a std::optional member and a TOML value doesn't have the key, the key-value will be std::nullopt. Similarly, if the struct's member holds std::nullopt, the conversion to TOML value will not include the key-value.

struct my_toml
{
  int a;
  std::optional<int> b;
};

TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(my_toml, a, b)

const toml::value v = toml::parse_str("a = 42");
const auto foo = toml::get<my_toml>(v);

assert(foo.a == 42);
assert(!foo.b.has_value());

[ToruNiina]

Great feature!

However, there is one thing I would like us to consider. I've left a comment, so could you check it?