SharedPtr.proxySwap

{
	SharedPtr!long a = SharedPtr!long.make(1);
	SharedPtr!long b = SharedPtr!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);
}