Make chunkset_avx2 half chunk aware

Make chunkset_avx2 half chunk aware

This gives us appreciable gains on a number of fronts.  The first being
we're inlining a pretty hot function that was getting dispatched to
regularly. Another is that we're able to do a safe lagged copy of a
distance that is smaller, so CHUNKCOPY gets its teeth back here for
smaller sizes, without having to do another dispatch to a function.

We're also now doing two overlapping writes at once and letting the CPU
do its store forwarding. This was an enhancement @dougallj had suggested
a while back.

Additionally, the "half chunk mag" here is fundamentally less
complicated because it doesn't require sythensizing cross lane permutes
with a blend operation, so we can optimistically do that first if the
len is small enough that a full 32 byte chunk doesn't make any sense.