Feature: variant2

[liss-h]

Adds variant2 a more efficient std::variant implementation for exactly two elements.
Should be a drop in replacement for std::variant<T, U> if I did not overlook anything.

POBR version increased because the layout of polymorphic_allocator with 2 allocators changed. But.. we are not using polymorphic_allocator anymore, so not POBR break?

TODO:

• add (unit) tests, is already indirectly tests through polymorphic allocator
• add example
• update readme
• determine if we consider this a POBR break

[mcb5637]

overall, looks good, just a few small issues.