TMI: rustpkg status

Today, I looked at the metabug for rustpkg and noticed that all of the items marked as "necessary to call rustpkg 'ready for use'" are finished. I added one more that was overlooked, which is workcache support, and after discussion with Graydon, added several more issues. Still, most of those should be pretty easy to fix, so we're very close to the point where we'll want to say to the community "hey, start using this and test!" Some people, of course, already have, such as cmr with rustdoc_ng. Which I'm currently trying to build on my system with rustpkg but am making rustc segfault. Must be Wednesday.