feat: add support for std::variant

[uyha]

No description provided.

[uyha]

@redboltz I'm having a bit of problem implementing object and object_with_zone, I'm not actually sure what they do and what operation is safe to do in the object case.

[codecov-commenter]

Codecov Report

Merging #1075 (35638ea) into cpp_master (ac062e2) will increase coverage by 0.11%.
The diff coverage is 96.96%.

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Additional details and impacted files

@@              Coverage Diff               @@
##           cpp_master    #1075      +/-   ##
==============================================
+ Coverage       85.63%   85.74%   +0.11%     
==============================================
  Files              79       80       +1     
  Lines            5040     5073      +33     
==============================================
+ Hits             4316     4350      +34     
+ Misses            724      723       -1     

[redboltz]

@redboltz I'm having a bit of problem implementing object and object_with_zone, I'm not actually sure what they do and what operation is safe to do in the object case.

object adaptor is only provided the packed type can be expressed by msgpack::object without zone allocation.

In the case of std::variant T is unpredictable. It is the same as std::optional.

msgpack-c/include/msgpack/v1/adaptor/cpp17/optional.hpp

Lines 67 to 81 in ac062e2

	template <typename T>
	struct object<std::optional<T> > {
	void operator()(msgpack::object& o, const std::optional<T>& v) const {
	if (v) msgpack::adaptor::object<T>()(o, *v);
	else o.type = msgpack::type::NIL;
	}
	};
	template <typename T>
	struct object_with_zone<std::optional<T> > {
	void operator()(msgpack::object::with_zone& o, const std::optional<T>& v) const {
	if (v) msgpack::adaptor::object_with_zone<T>()(o, *v);
	else o.type = msgpack::type::NIL;
	}
	};

std::optional adaptor provides object adaptor. If T has object adator, then forward it, otherwise compile error.
At first, I thought that std::variant is the same case, but I noticed that this adaptor needs index. So MessagePack ARRAY is required. That means zone allocation is required.

Hence, this std::variant adaptor shouldn't provide object adaptor.

[redboltz]

There are some of coding style fix request.
(Sorry for lack of documents)

I will add the comment inline later. Maybe we need to communicate several times.

[redboltz]

Please use 4 whitespaces indent.

I pointed out one case for each request. Please fix other same parts.

[uyha]

@redboltz could you please have a look again?

[redboltz]

I've checked updated one.

[uyha]

@redboltz are this PR and #1076 good enough for merge?

[redboltz]

Merged. Thank you!