TMI: Countdown to 0.6

We're down to 46 open issues for the release! I spent most of my time today working on #4439, which may be behind a few other open bugs. Bugs that cause the compiler to generate seg-faulting code are bad, so I figured I would see if I could fix it before the release. Turns out trait methods that take their self by-move... are implemented strangely. As usual, many bits of undocumented code in trans (the Rust-to-LLVM pass) combine to create confusion.

I thought the best way to go forward would be to convert the data structure representing the self object in trans to use a Datum -- a type that wraps up an LLVM value with data about whether it's meant to be passed by reference or by value -- instead of a raw value. I still think it is, but getting everything consistent means debugging many LLVM assertion failures.