Merge: Clean nit compilation directory

This PR cause the compilation directory to be removed after compilation.

This auto-removal is disabled if `--compile-dir` or `--no-cc` is used because it is a sane default I think.
Moreover, the compilation directory is renamed by defaut to `nit_compile` because there is no more point to make it hidden since its presence means an active request (`--no-cc`).

The PR is a bit premature since the boostrap will fail since `rmdir` requires the light FFI, so it is planned to be merged after the regen of c_src.

Close #792

Pull-Request: #1415
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>

diff --cc src/compiler/abstract_compiler.nit
Simple merge