Feature request: Deadline version

Requesting an associated init for `Delay` that sets it with an `Instant` instead of a `Duration` (and completes instantly if the `Instant` is in the past)

Start by locating the `Delay` type, its existing `Duration`-based initialization, and related tests. Check how `Instant` is represented and used in the project; done means an associated initialization path accepts an `Instant`, completes immediately for past instants, and has coverage for both past and future deadlines.

Written by the indexing model from the issue text.