Hide impl_data inside struct Dispatcher instead of using opaque pointers in struct Router

[oduortoni]

Hide impl_data inside struct Dispatcher instead of using opaque pointers.

Currently, impl_data is a purely internal structure for const Dispatcher. Instead of exposing it as a void pointer (void *). We should hide it inside the struct Dispatcher.

Benefits

1. Eliminate opaque pointers: the compiler can help catch errors at compile time.
2. Maintain type safety: casting struct RegexDispatcher* to struct Dispatcher* only risks alignment issues, which can be fixed with:

#include <stddef.h>
max_align_t __dummy_field_for_alignment;

3. Cleaner design — base Dispatcher handles the interface, specialized dispatchers manage their own data internally.

This approach allows internal data to remain private while still benefiting from strong typing and compiler checks.