Overhaul QML GC to support platforms where pointers use more than 48 bits

QML's gc (and general memory management) relies on NAN tagging to avoid boxing doubles and to quickly categorize what is inside of a QV4::Value.

NAN tagging breaks however on recent Android versions where the top-byte of pointers is tagged, and we need to store more than 48 bits. For now, Android can be configured to not rely on this. However, this might change already in the near future.

Thus, we need to change to a different approach in QML.

Without this change, QML applications will be broken on Android in the near future.