Revert "Revert "sort: improve average quicksort performance""
This reverts commit 30b87bb9.

See https://golang.org/cl/15688 for the CL being replayed.

Change-Id: I2ba36334003f4971f95a10536adfdd86be9a50de
Reviewed-on: https://go-review.googlesource.com/17389
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>