These compatibility changes:
- Use runtime.currentMSecs for the Clock timer "now" value
- Start executing hats before other threads change values
- Update test and fixtures to work with earlier hat execution
- Add test for hat execution block order
Threads are now removed after every "inner" step. Any thread that
reaches its DONE conditions is immediately removed instead of requiring
an extra step to remove it. As such tests that check the number of
threads have been updated to consider this.
