Add move constructors

[Reedbeta]

• Move constructors and move assignment now supported on Str and its subclasses.
• For local-buffered strings, does a copy; for heap-allocated strings, takes over the allocation from the move source; for unowned strings, points to the same memory as the move source.
• Allows using Str and friends in containers like std::vector, without incurring extra allocations when the container is resized.