Merge: Safe call operator

A long time ago, there was a proposal to have a safe call operator in Nit #1274 `x?.foo` that executes the call if `x` is not null and returns null otherwise (instead of aborting).

This was refused because at the time, the syntax `x?.foo` was considered weird and not POLA.
Moreover, what was proposed was a more general version of the concept that could be used everywhere, not only as a receiver that made the semantic quite complex and even less POLA.

Nowadays, most languages have adopted it https://en.wikipedia.org/wiki/Safe_navigation_operator so the syntax and behavior is not as weird as before.

This operator is nice in the context of Nit with nullable types since if avoids to use chained `if x != null then x.foo` and plays nicely with the type system.

Currently, only explicit dotted calls are handled; e.g. `x?.foo`, I'm not sure if something should be done with other call constructions like `x + y` or `x[y]`.

Pull-Request: #2761
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

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

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