simdarray: implement & test fusedMultiplyAdd()

refs #613

Signed-off-by: Matthias Kretz <kretz@kde.org>
move randomEngine definition to the top

Signed-off-by: Matthias Kretz <kretz@kde.org>
mark horizontal reduction functions as pure

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