Make boost::optional<T> trivially destructible, copyable and movable if T is

[1zeeky@…]

Conceptually there's nothing preventing boost::optional<T> from being trivially destructible, copyable and movable if T is. This could be useful, for instance, to use an boost::optional<T> as a union-member.

I have a POC implementation based on Boost 1.64 ​here.
I moved m_storage and m_initialized to a new class optional_storage, representing the new base class. Some methods managing those members are move there as well. optional_base indirectly inherits from optional_storage through optional_destroyer, optional_mover and optional_copier, which are templated classes including a boolean parameter to determine whether T is trivially destructible, movable and copyable and use template specialization to provide the respective trivial members.
The implementation relies on defaulting constructors, so without C++11 or newer only trivial destruction is possible.