Add utilities for measuring an effect's execution time

Something of the sort:

```scala
//in IO[A]

def timed: IO[(FiniteDuration, A)]

//in GenTemporal syntax

def timed: F[(FiniteDuration, A)]
```

After it lands in series/3.x, I think it would be a welcome addition to CE2 as well (for effects that have a `Clock[F]`, maybe - it would require a new syntax class).