Negative timestamp violate invariants

When adding negative timestamps into version vectors implicitly assumed invariants get violated. E.g. inserting a negative timestamp into an empty vv, yields an empty vv.

Suggested fix: We should either use UInt, or maybe start the counter in the SimpleEnv at Int.MIN_VALUE to increase the number of possibly generated Timestamps and fix the version vector accordingly.

Check the property tests for failing test cases. (We should probably also add a unit test case to cover negative counter values.)