Add tests to ensure types correctly use allocator_traits<A>::pointer, ideally based on boost::offset_ptr

[Twon]

As @Quuxplusone points out on this PR there should be further testing added for testing to ensure types correctly make use of allocator_traits<A>::pointer.

Adding Boost as a dependency is a little heavy weight as we require a mechanism for supporting the dependency (perhaps Conan?). However, we could emulate this with a custom offset_ptr implementation for the tests.

[jwakely]

This certainly doesn't work with fancy pointers:

value_types/experimental/polymorphic_inline_vtable.h

Line 77 in a7fa0db

auto* mem = cb_alloc_traits::allocate(cb_alloc, 1);

allocate and deallocate need to work with the allocator's pointer type, but construct and destroy work with raw pointers. You need to use std::to_address and std::pointer_traits<P>::pointer_to to convert between them.

[jwakely]

Same problem here:

value_types/polymorphic.h

Lines 98 to 100 in a7fa0db

	auto mem = cb_alloc_traits::allocate(cb_alloc, 1);
	try {
	cb_alloc_traits::construct(cb_alloc, mem, std::move(u_));

[Twon]

Note swap methods require the 2 phase swap to support fancy pointers