renameio.WriteFile writes files atomically by renaming temporary files.

See the subsequent changes for usage examples.

Updates #26794
Updates #22397

Change-Id: I4bfe3125a53f58060587f98afbb4260bb1cc3d32
Reviewed-on: https://go-review.googlesource.com/c/146377
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Russ Cox <rsc@golang.org>