Multi index fixes

With the previous version, most printers won't work in 7.0. I now fixed this, the problem is that in 7.0 I cannot subclass gdb.Value, so I'm using a wrapper, and that one was incomplete.

I also added test cases for the other printers. They all work except for intrusive list. I checked, and it was broken before my changes. I don't know how to test intrusive tree.

Otherwise the test scripts run fine in 7.0, 7.4 and 7.6. I included sample example outputs.