Improve enable_allocator_from_this

[alandefreitas]

The problem

enable_allocator_from_this is used to store the allocator with EBO. Its interface should be improved to reflect the proper API for EBO.

Proposed solution

https://github.com/alandefreitas/futures/blob/master/include/futures/detail/utility/maybe_empty.hpp

Alternatives I've considered

No EBO won't work for our classes. Maintaining the current API is not safe.

Additional context

EBO makes the containers 8 bytes smaller when the allocator is not stateful, such as std::allocator.