Reduce memory usage for object literals with integral keys

Be more aggressive about using sparse arrays for object literals than for
regular array handling, to reduce the memory usage in this use case. The
current heuristic is that indices over 0x1000 switch to sparse, which means a
simple { "3000" : "test" } creates a long vector instead of a sparse array.

Task-number: QTBUG-36803
Change-Id: Id05c76f0f597cd016114cd27ec54effbebe26e48
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>