Run cargo add in c2rust-instrument --set-runtime to add c2rust-analysis-rt automatically

[kkysen]

Currently, in #554, the c2rust-analysis-rt runtime must be added as an (optional) dependency in the instrumented crate. This runs cargo add c2rust-analysis-rt --optional so that it is automatically added and up-to-date when instrumenting. This also adds an optional --runtime ${runtime} argument to c2rust-instrument. If it is given, then cargo add c2rust-analysis-rt --optional --path ${runtime} --offline is run. Thus, it works well both when c2rust-analysis-rt is already on your machine and you're developing locally, as well as a user who would download c2rust-analysis-rt from crates.io. Note that cargo add is only run when --set-runtime is passed, so this is off by default, and must be opted-into.

Another, more complex, option is to copy the Cargo.toml to a Cargo.instrument.toml, add the c2rust-analysis-rt dependency there, and the run with --manifest-path Cargo.instrument.toml. However, that's more complex, and not all cargo commands uniformly take a --manifest-path argument, and there's no environment variable for that, which would be seamlessly inherited.

[rinon]

I don't think we should automatically run cargo add when instrumenting, that seems too magic to me. We should just document this requirement in a README. The most automatic thing I think we should possibly do is diagnose this error when instrumenting and present a friendly version.

[kkysen]

It's off by default now and enabled with --set-runtime. I still think this is good for testing especially, since it ensures the runtime crate is up-to-date and points to the correct version. This allows us to avoid keeping the runtime dependency in Cargo.toml checked into version control, too.