Internal redirects

[aturon]

Since middleware and endpoints run strictly after routing (by design), we may want to provide an ergonomic way of internally redirecting to other routes.

Some considerations:

• How best to express this? E.g. explicitly re-enter the router? Return a revised request? Something else?

• How should middleware be applied when handling the redirected request?

[aturon]

cc @wycats

[yoshuawuyts]

We still don't support this currently. Incorrectly closed by #450.

[yoshuawuyts]

A possible API here could be:

async fn endpoint(req: Request<State>) -> tide::Result {
    Ok(req.redirect("/other/route").await?)
}

And then inside the redirect submodule we could implement redirect::internal similar to the other methods for a non-status code based redirect.

As far as the impl goes we should call the router again at the top-level, and then returning the response again. Either we pass a reference to the router along (preferable) or we can pass a bi-directional channel (less preferable).