Add atomicWriteFile

[Bodigrim]

Now that we have openTempFile, it should be straightforward to implement atomicWriteFile as in atomic-write, which works by creating a temporary file and then moving it to the final location.

Maybe it should even be the default mode, replacing existing writeFile?..

[Bodigrim]

@hasufell what do you think?

[hasufell]

Looking at https://github.com/stackbuilders/atomic-write/blob/main/src/System/AtomicWrite/Internal.hs I come to the conclusion that it's not atomic.

It uses renameFile, which isn't atomic on windows: haskell/directory#109

Further, it allows to rename over existing files, which is even less atomic (also runs a doesFileExist and other things to get file permissions).