Change data collection for debugging to use QV4::Value.

Change data collection for debugging to use QV4::Value.

This patch changes the variable collection to store QV4::Value values
into a JS array, which is retained by the collector. This prevents any
GC issues, and gives a nice mapping from handle (used in the debugging
protocol) to JS value. It also allows for easy "shallow" object
serialization: any lookup can start with the QV4::Value, and add any
values it encounters to the array.

Testing is changed to use this collector directly, thereby testing the
class that is actually used to generate protocol data.

Task-number: QTBUG-47061
Change-Id: Iec75c4f74c08495e2a8af0fedf304f76f8385fd7
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>