Consider updating Allocator Access to add rvalue reference overloads

[cmazakas]

Right now most of allocator access takes the allocator by lvalue reference.

I presume this is to support stateful allocators.

Instead, I purport that allocator access should also work with temporarily materialized allocators.

This is useful in the case where we're rebinding a stored allocator and wishing to do a quick one-off allocation in a single place.

The goal is to remove the need to declare a stack-local, for example.

See the relevant CE example here: https://godbolt.org/z/hqKTGb3ab

Obligatory inline copy:

#include <boost/core/allocator_access.hpp>
#include <memory>
#include <type_traits>

namespace boost
{

template<class A>
allocator_pointer_t<std::decay_t<A>>
allocator_allocate(A&& a, allocator_size_type_t<std::decay_t<A>> n)
{
  return allocator_allocate(a,n);
}

}

auto tester(std::allocator<int> a) {
  // feed the rebound allocator directly using the converting constructor
  //
  return boost::allocator_allocate(std::allocator<float>(a),16);

  // want to avoid the need to declare `oa` to use the rebound allocator type
  //
  // std::allocator<float> oa(a);
  // return boost::allocator_allocate(oa,16);
}