Move rune benches into rune.

[udoprog]

So we almost have all the support we need for benchmarks with the #[bench] attribute in order to replace all benchmarks that are currently written in Rust.

What's missing is the following:

• [ ] Rune benches need to generate a compatible output.txt so that they work with https://rune-rs.github.io/rune/dev/bench
• [ ] Rune benches need to process and present results better, so that among other things outliers are filtered and they can be presented in a manner similar to with cargo bench (or criterion).