TMI: rustpkg: landed!

My pull request from yesterday landed, so I'm on to further cleanup. First, I submitted another pull request to use a condition for error handling instead of making a function return an Option. Conditions are supposed to be for rare errors; this particular case was the situation where creating the build directory in the package directory to put the build output in fails. I figured that would be pretty rare. I hadn't really written code with conditions before, but they're about as simple as it gets. Sort of like exceptions, but simpler and implemented in terms of Rust macros.

For my next trick, I wanted to make sure that rustpkg could compile a package that doesn't have a package script. For realism, I decided to see which of the Servo submodules had a relatively simple Makefile, meaning that rustpkg should be able to build it as-is; I picked rust-core-foundation, and am working on getting it to build now.