Making use of the same feature added in RefCount.
To keep with the intention of avoiding the allocation of "empty" array
headers, this introduces an unsharable_empty, which allows users to
maintain the "unsharable bit" on empty containers, without imposing any
actual allocations.
(Before anyone asks, there is no point to a zero-sized capacity-reserved
container so no other combinations are needed for now.)
Change-Id: Icaa40ac3100ad954fdc20dee0c991861136a5b19
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
3d61c5ca8f