implement built-in Clone

[RalfJung]

I was so frustrated with miri not building that I came up with a grand hack. The hack works, but unfortunately (and somewhat unsurprisingly), it uncovered that miri has already bitrotten: The Clone trait is now built-in, and miri can't handle that. It doesn't build, and once I got it to build, some tests fail. I stared at this stuff for 5min and still don't have the slightest clue of anything. Hopefully someone who actually knows more than absolutely nothing about trait resolution can chime in and fix this.^^