Rayon support

[DenialAdams]

Support parallel iteration via a new rayon feature. Fixes #98.

AI Disclosure: I prototyped this change with codex. When preparing this PR, I rewrote the change by hand to better match the existing codebase and improve on a few things. I had codex review the final change.

Implementation notes:

• The dep: syntax in Cargo.toml requires Rust 1.60. Alternatives seemed ugly, so I bumped the MSRV slightly.
• Cargo.lock was updated via cargo +nightly update -Z direct-minimal-versions
• Supported methods: par_iter, par_iter_mut, into_par_iter, par_keys, par_values, par_values_mut
• I skipped supporting HopSlotMap since it is deprecated. I skipped SparseSecondaryMap for now since it has more complicated internals and I don't personally have a use-case for it.
• I implemented ParallelIterator for the SlotMap and SecondaryMap iterators. For DenseSlotMap we can also implement IndexedParallelIterator as the iteration maps 1:1 with the underlying data.
• I tried to match the structure of the sequential iterators where possible. In some cases, a slightly different structure allowed for relaxed trait bounds on the parallel iterators and so in those cases there is some deviation.

Let me know if there is anything you'd like me to change.

Thanks you!