This patch constructs and loads the variant's contained value directly in place, instead of loading it outside and then moving it into place. This saves a move construction/assignment and thus benefits the case where moves aren't cheap (e.g. large POD). This also allows the signature and recursion logic of variant_detail::load_variant to be slightly simplified, since only the index is needed, not the type (the variant itself can figure out the type from the index).

This is basically the same thing as #698 but applied to variant.