Add a SimpleArrayData class

Add a SimpleArrayData class

This makes the ArrayData class 'pure virtual'. SimpleArrayData
now contains the implementation of simple arrays. This makes the
separation between simple and sparse arrays a lot cleaner.

It also allows us to move len and offset from the base class into
the SimpleArrayClass. This fixes some bugs where we accessed len
for sparse arrays leading to some buggy behavior.

Added a virtual length() method to ArrayData to query the highes
used index in the Array.

Change-Id: Iab2ba2a48ebe5b7031759eeb4ebe02b4d86233f0
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>