Remove `later` in favor of `sleep`

[natefaubion]

The signature of later no longer serves any semantic purpose. I think we should remove it in favor of sleep :: Int -> Aff eff Unit.

[no-longer-on-githu-b]

Data.Time.Duration.Milliseconds?

[nwolverson]

Is sleep the correct name? Not to say it's not, but it might suggest a synchronous sleep, another term that is used for an async wait is delay.