Decrease casting in rendcache functions

During work on legacy/trac#17776 (moved) i have identified three functions which could, with a type signature change, save some casting throughout the Tor code base. These functions are

void rend_cache_intro_failure_note(rend_intro_point_failure_t failure,
                                   const uint8_t *identity,
                                   const char *service_id);
STATIC int cache_failure_intro_lookup(const uint8_t *identity,
                                      const char *service_id,
                                      rend_cache_failure_intro_t
                                      **intro_entry);
STATIC void cache_failure_intro_add(const uint8_t *identity,
                                    const char *service_id,
                                    rend_intro_point_failure_t failure);

The identity pointers to uint8_t are internally cast to pointers to char for further processing. Meanwhile the callers of these functions cast the identity pointer from pointer to char to pointer to uint8_t to conform to its type signature.

Changing the type of the identity pointer from uint8_t to char would remove the need for casting back and forth.