allow passing by non-copying value

[andrewrk]

closes #733

Here's the new thing you're allowed to do:

test "pass by non-copying value" {
    assert(bar(Point{ .x = 1, .y = 2 }) == 3);
}

const Point = struct {
    x: i32,
    y: i32,
};

fn bar(pt: Point) i32 {
    return pt.x + pt.y;
} 

See #733 (comment)

All the old code still works, but it is now un-idiomatic when one would be better using these semantics.

And then the breaking change will come when we remove the implicit cast from T to *const T.