Add helpers for sorting slices.

Slice sorts slices:

    sort.Slice(s, func(i, j int) bool {
        if s[i].Foo != s[j].Foo {
            return s[i].Foo < s[j].Foo
        }
        return s[i].Bar < s[j].Bar
    })

SliceStable is the same, but does a stable sort.

SliceIsSorted reports whether a slice is already sorted.

Fixes #16721

Change-Id: I346530af1c5dee148ea9be85946fe08f23ae53e7
Reviewed-on: https://go-review.googlesource.com/27321
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>