Does this mean that `HashBuckets` how crashes and mergers no longer work for mutable hash sets?

`FlatHashTable` was refactored in 2.11 it seems.
We should either get this to work here, or not switch to 2.11.

Ensuring proper interaction with the stdlib inevidable leads to these kinds of problems. If we want to support 2.11, we should have a separate branch for 2.11, where this is properly fixed, and maintain that branch separately.

Orthogonally, we should have our own custom hash table implementations which are specialized, and support both mutable hash sets and mutable hash maps. These data-structures are used often enough to support our own, superior implementations. We should put this on a todo list.