Merge: null as receiver

Accepts literal `null` as a receiver of `==`, `!=` and `is_same_instance`.

This could help to finish #1041

Does people need other methods of Object available for `null`?

Pull-Request: #1082
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Etienne M. Gagnon <egagnon@j-meg.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

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

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

diff --cc src/semantize/typing.nit
Simple merge