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.

Libraries/LibCore/CArgsParser.h

@@ -31,9 +31,11 @@
 #include <AK/Vector.h>
 #include <stdio.h>
 
-class CArgsParser {
+namespace Core {
+
+class ArgsParser {
 public:
-    CArgsParser();
+    ArgsParser();
 
     enum class Required {
         Yes,
@@ -83,3 +85,5 @@ private:
 
     bool m_show_help { false };
 };
+
+}