nans2left!, nans2right!: be stable on both halves – NaNs and non-NaNs.

Previously, while this was leaving NaNs in order, as is necessary for
the algorithm to work correctly, but was always moving the first non-
NaN value to the opposite end of the array. This is *very* bad for the
median-of-three quicksort pivot selection algorithm when applied to a
pre-sorted array since the first and last elements will both end up
being very small, effectively always picking a pessimal pivot. This is
the root cause of the O(n^2) regression that @illericus was seeing.