MIC: add is_vector/mask traits

Signed-off-by: Matthias Kretz <kretz@kde.org>
MIC: simd_cast stub header

Signed-off-by: Matthias Kretz <kretz@kde.org>
traits: new is_atomic_simdarray/mask trait

An atomic simdarray type is one with a single Vector<T> member.

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