Add integration with Paralight

[gendx]

Paralight is a lightweight parallelism library tuned for indexed structures such as slices. Given that the internal representation of hashbrown's hash tables is a slice of buckets (that each optionally contain a value), it's a good fit to integrate with (https://github.com/gendx/paralight/issues/5).

This pull request is here to iterate on the design. As the integration needs access to the raw hash table representation, it's done here in the hashbrown crate (similarly to Rayon's integration).