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.

DevTools/HackStudio/ProjectFile.cpp

@@ -32,8 +32,8 @@ const GTextDocument& ProjectFile::document() const
 {
     if (!m_document) {
         m_document = GTextDocument::create(nullptr);
-        auto file = CFile::construct(m_name);
-        if (!file->open(CFile::ReadOnly)) {
+        auto file = Core::File::construct(m_name);
+        if (!file->open(Core::File::ReadOnly)) {
             ASSERT_NOT_REACHED();
         }
         m_document->set_text(file->read_all());