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/CUdpSocket.h

@@ -29,12 +29,16 @@
 #include <AK/Badge.h>
 #include <LibCore/CSocket.h>
 
-class CUdpSocket final : public CSocket {
-    C_OBJECT(CUdpSocket)
+namespace Core {
+
+class UdpSocket final : public Socket {
+    C_OBJECT(UdpSocket)
 public:
-    virtual ~CUdpSocket() override;
+    virtual ~UdpSocket() override;
 
 private:
-    CUdpSocket(int fd, CObject* parent = nullptr);
-    explicit CUdpSocket(CObject* parent = nullptr);
+    UdpSocket(int fd, Object* parent = nullptr);
+    explicit UdpSocket(Object* parent = nullptr);
 };
+
+}