Let's implement part of the std::shared_ptr interface:

• custom deleters
• weak pointers
• aliasing
• std::make_shared
• no allocators
• no std::atomic<std::shared_ptr>
• no std::enable_shared_from_this
• no std::make_shared_for_overwrite

Everything is in namspace ptr:

• class ptr::Shared
• class ptr::Weak
• ptr::make_shared
• ptr::swap