Update to rebased rustc-rayon 0.4
In rayon-rs/rayon#938, miri uncovered a race in `rustc-rayon-core` that had already been fixed in the regular `rayon-core`. I have now rebased that fork onto the latest rayon branch, and published as 0.4. I also updated `indexmap` to bump the dependency.
`Cargo.lock` changes:
Updating indexmap v1.8.0 -> v1.8.2
Updating rayon v1.5.1 -> v1.5.3
Updating rayon-core v1.9.1 -> v1.9.3
Updating rustc-rayon v0.3.2 -> v0.4.0
Updating rustc-rayon-core v0.3.2 -> v0.4.1