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.

Applications/FileManager/FileUtils.cpp

@@ -37,7 +37,7 @@ namespace FileUtils {
 
 int delete_directory(String directory, String& file_that_caused_error)
 {
-    CDirIterator iterator(directory, CDirIterator::SkipDots);
+    Core::DirIterator iterator(directory, Core::DirIterator::SkipDots);
     if (iterator.has_error()) {
         file_that_caused_error = directory;
         return -1;
@@ -104,7 +104,7 @@ bool copy_directory(const String& src_path, const String& dst_path)
     if (rc < 0) {
         return false;
     }
-    CDirIterator di(src_path, CDirIterator::SkipDots);
+    Core::DirIterator di(src_path, Core::DirIterator::SkipDots);
     if (di.has_error()) {
         return false;
     }