Updated rustc

[dmacks]

We currently have rustc-1.30.0, which is from late-2018. Some current releases of other packages are requiring newer versions (upstream is now at 1.56.0), so this is a blocker for updating them. The build process (at least as of our existing package) involves downloading a precompiled boostrap and then recompiling itself, or something like that. If anyone has experience with that sort of package, or some time to figure out how to update it, that'd be super.