Reduce the memory used by QStringHash

Reduce the memory used by QStringHash

This change reduces the memory consumed by individual QStringHashNodes,
and adds support for "linking" one hash to another that allows us to
avoid copying all the nodes.

Change-Id: Ib5bd151d8ec610a020fd125f46a4e218d959529b
Reviewed-by: Martin Jones <martin.jones@nokia.com>