The null-aware arrow operator: ?->

Instead of:
    if(ptr)
        ptr->func();
Write:
    ptr?->func();

The very bad:
    if( ptr && ptr->getSub() )
        ptr->getSub()->func();
Becomes:
    ptr?->getSub()?->func();

Support for the "default value if ptr is null" use case:
    n = ptr?->get_value() : 42;
Instead of:
    n = ptr ? ptr->get_value() : 42;

The three-way comparison operator <=> is 3 chars long; ?-> now seems
reasonable IMHO.

I'm unsure of all the possibilities for null-aware operators in C++,
but there's probably more to explore. I couldn't find anything in
relation to "null-aware" on the mailing list archive. "Null safety"
itself is a wider concept than null-awareness. If this was already
posted, sorry!

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