core_timing: Rename CoreTiming namespace to Core::Timing

Places all of the timing-related functionality under the existing Core
namespace to keep things consistent, rather than having the timing
utilities sitting in its own completely separate namespace.

src/core/core_cpu.cpp

@@ -93,14 +93,14 @@ void Cpu::RunLoop(bool tight_loop) {
 
         if (IsMainCore()) {
             // TODO(Subv): Only let CoreTiming idle if all 4 cores are idling.
-            CoreTiming::Idle();
-            CoreTiming::Advance();
+            Timing::Idle();
+            Timing::Advance();
         }
 
         PrepareReschedule();
     } else {
         if (IsMainCore()) {
-            CoreTiming::Advance();
+            Timing::Advance();
         }
 
         if (tight_loop) {