lib/standard: Fixed implementation of to_s in Int.

Previously, there was an issue when outputting an Int as a String, the %d was for ints, while a Nit int is a C long, which might be bigger depending on the architecture.

There will be warnings when compiling nitg until PR #457 is accepted and c_src is regenerated.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>