Re: add AVX2 support to simd.h

Here is what I have staged for commit.  One notable difference in this
version of the patch is that I've changed

    +    if (nelem <= nelem_per_iteration)
    +        goto one_by_one;

to

    +    if (nelem < nelem_per_iteration)
    +        goto one_by_one;

I realized that there's no reason to jump to the one-by-one linear search
code when nelem == nelem_per_iteration, as the worst thing that will happen
is that we'll process all the elements twice if the value isn't present in
the array.  My benchmark that I've been using also shows a significant
speedup for this case with this change (on the order of 75%), which I
imagine might be due to a combination of branch prediction, caching, fewer
instructions, etc.

-- 
Nathan Bossart
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