Support for build kinds (examples, benches, etc.)

[jordanisaacs]

I recently have finished up a working PR for build kinds in buildRustCrate.I have a branch up with a small commit that gets the crate2nix codegen in a state where you can manually edit the Cargo.nix to test out the PR. (and it is working fantastic, got criterion benches running for a crate). But manual edits aren't great, and I am now shifting my focus to getting a good API for it to play nicely with crate2nix. The goal is to get crate2nix to easily compile tests, benches, docs, etc.

I am open to ideas and help so starting an issue! And some extra eyes on the buildRustCrate code would be fantastic.