Implement Sharray ("shifted array")

[saxbophone]

• Contiguously stored (like array, vector)
• Uses std::allocator and std::allocator_traits (see: here and here)
  • Remember the golden rules: allocate(), construct(), destroy(), deallocate()!
• Fast insertion at front and back (like deque, list)
• constexpr-safe
• Memory-safe
• Unit tests