Release the VM of unused heap chunks

This changes the way that available items are linked together to
allow releasing chunks not containing any inUse item after being
swept. Instead of putting a freed item at the top of the stack and
make it link to the item previously at the top, relink all
items !inUse to point to the previously visited unused item during
sweeping.

This allows any chunk to be released in the process while making sure
that no previous or next item will link to an address in that chunk.

The performance penality of relinking the list has been measured
to be ~0.6% with v8-bench.js tst_QJSEngine::largeSweep. In exchange,
this helps keeping the amount of chunks to sweep lower after peaks
of heap allocation, even though that ability will be restrained by
heap fragmentation in long running applications.

Task-number: QTBUG-41099
Change-Id: I81a70f0a930b679a1bef47e630b23dab5f6d1218
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>