Add support for new types in C++17

[drrlvn]

• std::optional
• std::variant
• std::string_view

These have boost adaptors and need std ones as well.

[redboltz]

@spatz , thank you for the comment.
I think that std::optional and std::string_view should be supported by the new adaptors.
They are very simple.

However, std::variant is a little different. msgpack-c has the types msgpack::type::variant and msgpack::type::variant_ref. They are implemented using boost::varinat. That means msgpack-c doesn't provide boost::variant adaptor.
See https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_variant

msgpack::type::variant and msgpack::type::variant_ref can be implemented using std::variant instead of boost::variant. I think that it should be provided, and user can switch the implementation using some macro as MSGPACK_VARIANT_USE_STD.

[redboltz]

I implemented adaptors for std::optional and std::string_view. See #607.

msgpack::type::variant and msgpack::type::variant_ref implemented using std::variant is not included in #607.

msgpack::object is a composite data structure, so msgpack::type::variant reflects the structure. In order to implement it, I use boost::recursive_wrapper. However, std::variant doesn't have something like std::recursive_wrapper. Maybe I need to use the combination of forward declaration and std::unique_ptr<basic_variant>. It isn't simple boost to std replacement. I need to study more about it.

So I separated variant support from #607.

[drrlvn]

That's perfectly understandable.
Thank you very much for your responsiveness!

[redboltz]

Meged #607.
std::optional and std::string_view are supported on master.

[drrlvn]

Also, std::byte should be supported.

[redboltz]

std::byte is added by #719 .