TMI: More rustpkg work

Today's work on rustpkg is all in my pending pull request. I wrote out a schedule containing all the significant pieces-of-work remaining on rustpkg that Graydon and I could think of right now, and estimated times for each task (like all software engineers I'm terribly bad at estimating time for things). I'm hoping having everything laid out in a linear fashion will help me stay on-track, though.

I'm also glad to have spent some time writing down hypothetical-rust-package-commands-that-should-work (in a text file in the pull request), even though we can't do automatic testing yet. It's good to know what you're doing before you do it...

As an end-of-the-day small task, I made the typechecker able to survive more errors; generally the typechecker should be able to emit a type error and then keep going (since most people like to see as many errors as possible so they can try to fix them in one go, rather than one at a time) but there are a few cases here and there where it's tricky to set up the right data in various environments for the typechecker to keep going. So there's two less of those cases now.