Make a repository to hold our rust dependencies in a form Cargo can use

To get good jenkins builders deployed, as well as for other reproducibility reasons, we should get a repository that holds our rust dependencies in a form that our build can use, so that cargo doesn't need to fetch anything while the build is happening.

It seems that this is something that other Rust people care about, so it is possible that a better solution will arrive soon after we get something done here: https://internals.rust-lang.org/t/call-for-help-cargo-airplane-mode/6134/5