vala: Add support for null-safe access operator

vala: Add support for null-safe access operator

This implements support for the "null-safe" operator of C#
(also called "safe navigation" operator).

  var foo = bar?.foo;
  var foo = bar?.foo();
  var foo = bar?[3];
  var foo = bar?[1:2];

It applies to plain member access, method calls, element access and
slice expressions (including void method calls, which are simply bypassed
if the inner expression is null).

Fixes https://gitlab.gnome.org/GNOME/vala/issues/522