We've previously used functions in the Router type to provide
information for subrouters. But this accesses the Requests too
early, and breaks sharing of the router structure in general,
causing the Router or large parts of the Router to be recomputed
on every request.
We now do not use functions anymore, and properly compute all
static parts of the router first, and gain access to the request
only in Delayed.
This also turns the code used within Delayed into a proper monad
now called DelayedIO, making some of the code using it a bit
nicer.
* Improves how Routers are built, in particular via
the `choice` smart constructors. Static lookups are
now used more often.
* We now have test cases making sure that certain
routers have the same structure.
* The router structure can now be visualized for debugging
purposes as a tree. The new functions `layout` and
`layoutWithContext` do this.
