Use an array of Value's for Object::memberData

Use an array of Value's for Object::memberData

This cuts the memory required to store properties
in an object in half for the common case. Accessor
properties require two slots inside memberData,
but data properties only one.

Change-Id: I0bab1b88ca9ed5930abf065c77c89985b9ed5320
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>