Provide type alias for time.Duration

[rojer9-fb]

It would be nice to not have to import the "time" package at all.
It makes it easier, just by eyeballing the imports, to audit a package to ensure time.Sleep or time.After does not get used by accident.
However, in many cases the time package is required to use time.Duration.
It would be nice to have a type alias in the clock package, so that clock.Duration could be used interchangeably.