AK: Move Stream and SeekableStream from LibCore

`Stream` will be qualified as `AK::Stream` until we remove the
`Core::Stream` namespace. `IODevice` now reuses the `SeekMode` that is
defined by `SeekableStream`, since defining its own would require us to
qualify it with `AK::SeekMode` everywhere.

Userland/Applications/Spreadsheet/ExportDialog.h

@@ -27,7 +27,7 @@ struct CSVExportDialogPage {
         Preview
     };
 
-    ErrorOr<void> generate(Core::Stream::Stream&, GenerationType);
+    ErrorOr<void> generate(AK::Stream&, GenerationType);
 
 protected:
     void update_preview();