Minimal indexmap support for rustc-rayon

[cuviper]

Currently, indexmap has an optional "internal only" dependency on rustc-rayon to implement parallel iterator support for use in the compiler. This PR flips that around to have rustc-rayon implements those parallel iterators, so indexmap doesn't need to bother with this relatively niche use-case. If this is accepted and published, I intend to remove that "internal" dependency.

These implementations are all straightforward with the Slice API:

• impl<'a, K, V, S> IntoParallelIterator for &'a IndexMap<K, V, S>
• impl<'a, K, V, S> IntoParallelIterator for &'a mut IndexMap<K, V, S>
• impl<'a, T, S> IntoParallelIterator for &'a IndexSet<T, S>

However, indexmap/rustc-rayon also had by-value IntoParallelIterator implementations that are not feasible without direct access to their storage. Nothing in the compiler is currently using that though, so I think it's ok to drop it.

[cuviper]

For reference, here's how the impls currently look on the indexmap side:
https://github.com/indexmap-rs/indexmap/blob/31c9862f71e00411271acb7c9bea3a21b3d08880/src/rustc.rs

... and they use the forwarding macros found here:
https://github.com/indexmap-rs/indexmap/blob/31c9862f71e00411271acb7c9bea3a21b3d08880/src/macros.rs#L126

So those are using rayon's Vec/slice implementations and then mapping the private Bucket to the final Item type.