Merge: Some niti cleaning

Small and cosmetic fixes for the interpreter.

See the tentative to implement `String::files` with some (non-reflexive) metaprogramming.

Pull-Request: #971
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/interpreter/naive_interpreter.nit
Simple merge