Add "Benchmarks" section to Guide

[stewart]

The #[bench] attribute is currently unstable, and is only usable on nightly Rust.

However, being able to benchmark code with cargo bench is very useful functionality when writing performance-focused code, and something that the Cargo documentation currently glosses over.

Some useful information on how to get benchmark tests up and running, with necessary caveats about stability, would likely be very helpful to new users.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

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[steveklabnik]

Thanks for this! I'm kind of conflicted, as it's not totally clear how stable/unstable stuff should work in Cargo's docs. And about half of this is more about Rust than it is about the cargo bits. Which also isn't bad..... i dunno.

We certainly need to explain this somehow, though.

[bors]

☔ The latest upstream changes (presumably #3004) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

I'm gonna close this due to inactivity, but feel free to resubmit with a rebase!