Introduces the following features:
 - A frontend for `rouille`!
 - Custom enumeration type for several possible representations of url encoded
   parameters. This should make it possible for nearly every frontend to provide
   a zero-copy implementation of `WebRequest`.
 - Redesign of `WebRequest` and `OwnerAuthorizer` makes it possible to have
   value types as Requests and operate on them instead of `&mut`.

These interfaces were improved:
 - Documentation in `frontends` provides a guide for implementations of custom
   frontends.
 - A cleaner type design for `WebRequest::urlbody` and `WebRequest::query` moves
   the work for ensuring single values in queries further into the library and
   delays enforcing this restriction.
 - The `AuthorizationFlow` is more open as it returns the request initially
   provided when it is done. `OwnerAuthorizer` is no longer mandatory to drive
   this flow but remains a useful tool in some cases where it provides cleaner
   code.