Implement Debug for Schedule, Executor and other subtypes

[Rua]

This is an attempt to implement #198. I've followed the guidelines for the Debug trait relatively strictly, so it mostly shows the exact internal state of the various types. I could make some of it a bit more clean and easy to read, for example by grouping the values of Executor by index, or showing only the system names instead of the full SystemId, but I first wanted some feedback on whether such a deviation from standard Debug output would be ok.

I introduced an instance of unsafe in the SystemBox implementation. I don't know if that is sound, that should be verified by someone who knows better.

Perhaps implementing this using the Debug trait is the wrong approach here, I just went with that because the original issue described it that way.

[TomGillen]

I am sorry it has taken me so long to get to these PRs. These changes look good - I have suggested a note explaining why that unsafe should be OK.