RcPtr.proxySwap

{
    RcPtr!long a = RcPtr!long.make(1);
    RcPtr!long b = RcPtr!long.make(2);
    a.proxySwap(b);
    assert(*a == 2);
    assert(*b == 1);
    import std.algorithm : swap;
    swap(a, b);
    assert(*a == 1);
    assert(*b == 2);
    assert(a.useCount == 1);
    assert(b.useCount == 1);
}