New lint: swap_with_temporary

[samueltardieu]

This lint detects inefficient or useless {std,core}::mem::swap() calls such as:

    // Should be `a = temp();`
    swap(&mut a, &mut temp());
    // Should be `*b = temp();`
    swap(b, &mut temp());
    // Should be `temp1(); temp2();` if we want to keep the side effects
    swap(&mut temp1(), &mut temp2());

It also takes care of using a form appropriate for a () context if swap() is part of a larger expression (don't ask me why this wouldn't happen, I have no idea), by suggesting { x = y; } (statement in block) or {std,core}::mem::drop((temp1(), temp2()).

changelog: [swap_with_temporary]: new lint

Close #1968

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