Optimise NEON assembly for get_sad for widths 32, 64 and 128
Optimise NEON assembly for get_sad for widths 32, 64 and 128

Reduces execution time for these functions by between 35% and 42%.
Binary size of the SAD module is reduced by 23%, to less than 1kB.

Reorder widths 4 and 8 for consistency with other widths.

xiph/rav1e,xiph/rav1e