Refactor the webserver

[dhil]

The source code for the webserver looks like it was written in anger. It has grown into quite the mess which is difficult to extend. We should rewrite/refactor the webserver, and in doing so we should get rid of the mutual dependency between the webserver and evaluator (e.g. by hoisting the routing table out of webserver.ml). We should also make sure that the webserver is oblivious to the prelude/global bindings, which is currently not the case.