Interval types

The windowing and time-restriction APIs are begging for interval types instead of the current haphazard mix of tuples and argument positions.  There could even be an infix unicode operator.  Then windowing could simply be written as `sig[(-200μs ↔ 800μs) + [1s,2s,3s]]`.  

Some questions:
- Is there a good way to represent open-ended intervals?
- Is this too magical?
