nix: Enabled tooling with cargo2nix

[jpds]

I did this as part of the testing for the LTO change and just making it easier to do builds across my laptop and test server.

Basically, one just needs to install Nix on their machine: https://zero-to-nix.com/start/install

...and then instead of developers having to bootstrap their own Rust (in this case)/C++/Python/JavaScript toolchains on their systems, they just run:

nix build

...the tool then fetches the required (and pinned) toolchain and performs the build of the project.

Debug builds can be done with:

nix develop # spawns a temporary shell session with the required tooling added
cargo build
cargo test

Just a thought as people mentioned that developer speed was important.