Add ItemOrder concept
This is a generalized contract for container-like classes, expressing how contained
items contribute to equality contract of the container.
The defined values are ItemOrder.Ordered and ItemOrder.Unordered. For convenience,
these are also exposed as ItemOrder.itemOrder(), reporting the implemented class
of ItemOrder.
JIRA: YANGTOOLS-1170
Change-Id: Ic67926c2146696ecb4be6f664720e842f363c6b0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
c0efc51f8eaec305de6524c8f98b30e20730feb7)