Add function for checking ranges are currently valid

In the CircuitHub codebase we have:
```haskell
-- | @range \@> now()@: true when the current timestamp falls within the range.
currentlyValid :: Expr (Range UTCTime) -> Expr Bool
currentlyValid range = rawBinaryOperator "@>" range now
```

we should upstream this or something similar, since this is something we want to quite often do with ranges.