Save memory on array data

Save memory on array data

Store a simple vector of Values in the array data,
instead of a Vector of Property's. This halfes the
memory consumption on 64bit and simplifies our code.

If an indexed property gets converted to an accessor
property, we simply convert the ArrayData into a
SparseArrayData.

Add support in SparseArrayData to allocate double slots
(two Value's) to hold a full Property in case someone
sets an accessor on an indexed property.

Some methods still return a Property*, but this is safe, as
only the first Value in the Property pointer will ever get
accessed if the Property doesn't contain an accessor.

Change-Id: Ic9b0f309b09a2772a328d947a10faaf3be9fe56f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>