This introduces a `Delayed` type in `RoutingApplication.hs` that
contains a handler together with delayed checks. There are several
blocks of delayed checks, so that we can ultimately execute them in the
order we desire.
The process is documented in more detail in `RoutingApplication.hs`.
This change adapt the auth combinator example to the new router code.
In general, the server interpretation of user-written combinators will
be affected by the new routing code.
The change here also introduces a change in functionality: previously,
wrong authentication triggered a "hard failure", whereas we now trigger
a "soft failure", which is recoverable. For the simple example, this
does not make a lot of difference.
In general, I think having a soft failure is the right option to take
here, although we want a more general story about the relative
priorities of different error codes.
