feature: parallel retain

In #82 a previous discussion was had, and a conclusion was that users could implement a parallel retain by using the raw shards() API.

However, with the switch to using hashbrown's RawTable, this no longer works -- RawTable doesn't have a retain API. You can still implement it by hand, but it now involves some unsafe and other fiddly bits.

What do you think about a PR to add a par_retain() method when the rayon feature is used? I'd be happy to contribute this.