Merge: compiler: compile_dir can be anywhere

Merge: compiler: compile_dir can be anywhere

Fix an old TODO. Now you can compile where you want and stop having a bunch of fat .nit_compile directories everywhere.

~~~sh
$ nitg --compile-dir /tmp/nit_compile examples/hello_word.nit
~~~

Pull-Request: #789
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>