V4: lower memory allocator pressure.

V4: lower memory allocator pressure.

Changes to datastructures and more re-using of locally used temporary
vectors. For the test regress-74474-002.js this lowers the total
allocated memory from 1.98GB to 158MB. Thse peak memory usage stays at
75MB. There is no functional change.

This should give a modest performance improvement which mainly depends
on the speed of malloc()/free().

Change-Id: I1877c1903e59a33ee79ff2b801ef6f2c1cee30a6
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>