feat(criterion): make `Bencher` implement `Send + Sync`
`criterion`'s `Bencher` implements `Send + Sync`, which allows running
the benchmark itself on another thread. This allows something like
`|b| /* my rayon thread pool */.install(|| b.iter(|| /* bench */))`.
fix(cargo-codspeed): use target.rustflags config so it's merged with the .cargo/config.toml
When using build.rustflags, it won't be merged because it has a lower precedence. See: https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildrustflags