`recursive_wrapper<T>` should not contain allocator type in its full signature

[saki7]

Currently, the full name of recursive_wrapper<T> is recursive_wrapper<T, std::allocator<T>> . This makes the variant's name extremely long in parser combinator applications because of the allocator type. This is a huge QoL problem and we should make the recursive_wrapper<T> a distinct type that has no allocator parameter.

This is quite non-trivial task because recursive_wrapper has many special handlings in rvariant, and rvariant's specification must be modified carefully to adapt the new interface.