traits: rename to is_simdarray

for consistency with the class name it's now called is_simdarray instead
of is_simd_array

Signed-off-by: Matthias Kretz <kretz@kde.org>
tests: fix IndexesFromZero usage

IndexesFromZero works with floating point vector types, too. Since I
want to move Vector::IndexType over to simdarray<int, Size> it is much
easier now to use that directly than the previously needed conversion
from an int_v to V.

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