Commit my work tree... This reintroduces the Reference type, but a more mean one -- it's basically a ProperySlot with extra whistles, and permits stuff like direct writes. The addition simplifies some code and gives me more flexibility in optimizing some things (but makes other things heavier).

IIRC, this is slightly faster overall, but I don't have numbers at hand; I'll probably have to refine this some more before it goes into trunk anyway.