Add regret.policies.ElapsedTime

This deprecation policy uses time-since-deprecation as the measure of when to remove an object (or permanently change it) once it has been deprecated.

Errors shown via #14 / #1 should be fired after the expected elapsed time has passed, and deprecations should require specifying the initial date.

Some concern may be paid towards the fact that typically deprecations will be performed some period of time before the version containing them is released. We can check what if anything projects that do deprecations in this way tend to do, which likely is either "ignore that fact if their releases are often enough", or add a fudge factor's worth of time onto the end, such that deprecations are removed after 13 months, say, instead of a year.