Kernel: Make self-contained locking smart pointers their own classes

Until now, our kernel has reimplemented a number of AK classes to
provide automatic internal locking:

- RefPtr
- NonnullRefPtr
- WeakPtr
- Weakable

This patch renames the Kernel classes so that they can coexist with
the original AK classes:

- RefPtr => LockRefPtr
- NonnullRefPtr => NonnullLockRefPtr
- WeakPtr => LockWeakPtr
- Weakable => LockWeakable

The goal here is to eventually get rid of the Lock* classes in favor of
using external locking.

Kernel/Time/PIT.cpp

@@ -16,9 +16,9 @@
 #define IRQ_TIMER 0
 namespace Kernel {
 
-UNMAP_AFTER_INIT NonnullRefPtr<PIT> PIT::initialize(Function<void(RegisterState const&)> callback)
+UNMAP_AFTER_INIT NonnullLockRefPtr<PIT> PIT::initialize(Function<void(RegisterState const&)> callback)
 {
-    return adopt_ref(*new PIT(move(callback)));
+    return adopt_lock_ref(*new PIT(move(callback)));
 }
 
 [[maybe_unused]] inline static void reset_countdown(u16 timer_reload)