Hook improvements (take 2) (#563)

Hi @tiann.

Thanks for the great project, I had great fun playing around with it.

This PR mainly tries to further minimize the possible delays caused by
KernelSU hooking.

There are 3 major changes:
- Processes with 0 < UID < 2000 are blocked straight-up before going
through the allow_list.
I don't see any need for such processes to be interested in root, and
this allows returning early before going through a more expensive
lookup.
If there's an expected breakage due to this change, I'll remove it. Let
me know.
- A page-sized (4K) bitmap is added.
This allows O(1) lookup for UID <= 32767.
This speeds up `ksu_is_allow_uid()` by about 4.8x by sacrificing a 4K
memory. IMHO, a good trade-off.
Most notably, this reduces the 99.999% result previously from worrying
milliseconds scale to microseconds scale.
For UID > 32767, another page-sized (4K) sequential array is used to
cache allow_list.

Compared to the previous PR #557, this new approach gives another nice
25% performance boost in average, 63-96% boost in worst cases.

Benchmark results are available at
https://docs.google.com/spreadsheets/d/1w_tO1zRLPNMFRer49pL1TQfL6ndEhilRrDU1XFIcWXY/edit?usp=sharing

Thanks!

---------

Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>

kernel/manager.h

@@ -15,7 +15,7 @@ static inline bool ksu_is_manager_uid_valid()
 
 static inline bool is_manager()
 {
-	return ksu_manager_uid == current_uid().val;
+	return unlikely(ksu_manager_uid == current_uid().val);
 }
 
 static inline uid_t ksu_get_manager_uid()