Loopsimd alignment

This MR adds a additional template parameter to LoopSIMD to specify the alignment.

The default alignment is the alignment of the underlying type.

I'm not sure how to determine the proper alignment for some return types or RebindType. To mitigate that problem I define conversion constructors and assignment operators from LoopSIMD types with different alignment.

This MR depends on !669 (merged).

Feel free to push at the source branch!