Reorganise into faster (R2) and finer (R3)

src/ext/kissfft/COPYING

@@ -0,0 +1,11 @@
+Copyright (c) 2003-2010 Mark Borgerding . All rights reserved.
+
+KISS FFT is provided under:
+
+  SPDX-License-Identifier: BSD-3-Clause
+
+Being under the terms of the BSD 3-clause "New" or "Revised" License,
+according with:
+
+  LICENSES/BSD-3-Clause
+