LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Userland/disk_benchmark.cpp

@@ -119,7 +119,7 @@ int main(int argc, char** argv)
             Vector<Result> results;
 
             printf("Running: file_size=%d block_size=%d\n", file_size, block_size);
-            CElapsedTimer timer;
+            Core::ElapsedTimer timer;
             timer.start();
             while (timer.elapsed() < time_per_benchmark * 1000) {
                 printf(".");
@@ -160,7 +160,7 @@ Result benchmark(const String& filename, int file_size, int block_size, ByteBuff
 
     Result res;
 
-    CElapsedTimer timer;
+    Core::ElapsedTimer timer;
 
     timer.start();
     int nwrote = 0;