Introduce rune bench for running micro benchmarks

[udoprog]

This allows for specifying benchmark functions with the #[bench] attribute, which uses a similar contract as the same attribute in Rust.

#[bench]
fn bench_try(b) {
    b.iter(|| {
        Ok(Ok(Ok(Ok(Ok(Ok(42))))))??????
    });
}

This can be run with the new bench subcommand. It's currently very simple and only reports average and standard deviation after performing a number of warmups and iterations. I expect this to be improved at a later point.

> cargo run --release --bin rune -- bench scripts\benches.rn
Found 1 benches...
bench bench_try#0: mean=780.00ns, stddev=50.99, iterations=100

I've re-used some of the plumbing introduced with the #[test] attribute in 4b4683a.