elm-repl derps in read-only directories

[bukzor]

A demonstration:

$ cd /
$ elm-repl
---- elm-repl 0.18.0 -----------------------------------------------------------
 :help for help, :exit to exit, more at <https://github.com/elm-lang/elm-repl>
--------------------------------------------------------------------------------
> 1
elm-repl: repl-temp-000.elm: openFile: permission denied (Permission denied)

If this were a bash script, I'd use one of ${TMPDIR:-/tmp} or $(mktemp -d).
Surely there's a haskell package that gives you a cross-platform tmpdir.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[bukzor]

I think this is what you want:

https://hackage.haskell.org/package/temporary-1.2.0.4/docs/System-IO-Temp.html#v:withSystemTempDirectory