Move only types in recursive_wrapper

[Kojoley]

Consider this simplified example:

#include <boost/variant.hpp>
#include <memory>

struct A {};
using ABC = boost::variant<
    boost::recursive_wrapper<std::unique_ptr<A>>
>;

int main()
{
    ABC a, b = std::make_unique<A>();
    a = std::move(b);
}

Using unique_ptr in the example as it a known non-copyable but movable type

This does not compile because variant tries to make a copy of unique_ptr. Assuming my knowledge (and variant knows this too) that recursive_wrapper contains a pointer to heap allocated object I was expecting that variant assignment will swap pointers (the assignment operator does swap pointers) or in the worst case it would move the object.

It would be even better if variant always perform on move pointer stealing from other variant with active recursive_wrapper, it is the main thing why spirit::x3::variant exists https://github.com/boostorg/spirit/issues/445#issuecomment-459355870

[Kojoley]

Even if #59 is not acceptable there is a way to fix this that way - variant move constructor default constructs a recursive_wrapper (in case of exception here simply no variant will be constructed and rhs is untouched) and then move assigns an rhs recursive_wrapper (we speaking about the case when move does not throw). From this change all nothrow move constructible recursive_wrappered types will benefit, because instead of making two copies it will default construct + move which usually should be a way more cheaper.