Allocator<T> now uses minimum alignment of the SIMD target

Allocator<T> just got more useful: e.g. Allocator<float> now allocates
aligned memory suitable for the default Vc::float_v aligned load/stores.
It uses hardcoded alignments for AVX and SSE - but that's all that Vc
0.7 supports anyway.

includes unit test that fails without the special handling in Allocator.

closes #616

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