Re-introduce Cargo concurrency in builds

[zec]

Right now, we run Cargo with --jobs 1 (see #14), as there doesn't seem to be a way to expose the Make jobserver to the cargo invocation from CMake. It would be nice to enable such exposure to be able to exploit concurrency within a Cargo build in a controllable manner.

Maybe alter the cFS CMake framework so that you can use Ninja?