reduce the number of arguments in the Dispatcher interface (struct) funtion pointers

[oduortoni]

We are passing so many arguments to each function pointer yet we can just pass struct Router that contains everything we need.

Simplifies to:

struct Dispatcher {
        HandlerFunc (*match)(struct Router* router, Request* req);
        void (*add_route)(struct Router* router, const char* pattern, HandlerFunc handler);
        void (*mount)(struct Router* parent, const char* prefix, struct Router* child);
        void (*free)(struct Dispatcher* self);
};

Once again, eliminating opaque pointers while still staying flexible enough.