Tuples: changed tuple.ToString() to only add a trailing ",)" for singleton only to be more in line with Python

- The trailing ',' should only be added for singleton, so that we can more easily distinguish between integer (1) and singleton (1,)
- For the others "(1, 2)" is enough to parse it as a tuple
Tuples: packing/unpacking of floats and doubles

- finished implemention of unpacking of floats and doubles
- made it work with FdbKey.PrettyPrint(...)
- added some unit tests
Tuple: attempts to make FdbKey.Dump() not trigger unwanted breakpoints in debug mode

- not sure if DebuggerNonUserCode will help or not...
- added comments so that people walking up the stack will maybe figure out what is going on