Beautiful Code — go sort

Beautiful Code — go sort sort: optimize average calculation in symMerge and doPivot. Change code of the form `i + (j-i)/2` to `int(uint(i+j) >> 1)`. The optimized average calculation uses fewer instructions to calculate the average without overflowing at the addition. Analogous to https://golang.org/cl/36332. name old time/op new time/op delta StableString1K-4 49.6µs ± 3% 49.1µs…