all: use sort.Slice where applicable

I avoided anywhere in the compiler or things which might be used by
the compiler in the future, since they need to build with Go 1.4.

I also avoided anywhere where there was no benefit to changing it.

I probably missed some.

Updates #16721

Change-Id: Ib3c895ff475c6dec2d4322393faaf8cb6a6d4956
Reviewed-on: https://go-review.googlesource.com/30250


TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>