simd_cast: generic simd_cast_interleaved_argument_order

The next test required even more function arguments. And I finally had a
somewhat simpler idea to implement the argument deinterleaving
generically. Now simd_cast_interleaved_argument_order works for any even
number of arguments.

(with documentation)

Signed-off-by: Matthias Kretz <kretz@kde.org>
dox: new script to build internal docs

Signed-off-by: Matthias Kretz <kretz@kde.org>