Merge: Interpreter FFI: custom compile_dir, use /tmp/ by default, and delete after...

Generate native files in the `/tmp/` directory and delete them after execution. The directory can be customized using `--compile-dir`.

This should fix #1974 and #1975.

To support some other OS, we will need to use something other than `getpid` and `/tmp/`. We could also improve upon this by reusing the generated libraries between executions.

Pull-Request: #1976
Reviewed-by: Jean Privat <jean@pryen.org>