Introduce `SendFuture::repeat_every`

[thomaseizinger]

Extracted out of #71.

This should be introduced via an extension crate to not grow the core API of xtra unnecessarily.

It is important that this must work only on Handlers that return (). That is also the main reason for providing this functionality to users and not telling them to write it themselves. It is a potential footgun to drop that return value.

[Restioson]

This sounds like a nice API! Though, as we are currently moving toward removing convenience methods to slim down the API with a view to an extension crate soon, then this might be an odd addition to merge now

[thomaseizinger]

Fair point. Would you be okay with removing the feature entirely and adding it back later once the dust around the slimming down has settled?

[Restioson]

Sure, that makes sense!