AHashMap, HashMap and HashSet have inconsistent field and variable names - not only between themselves, but also internally, and across the codebase.
When I personally tried to understand the code, this threw me off many times, and made the classes extremely hard to understand and compare.

I've renamed variables to be consistent. This should help these classes be more maintainable.

Change List

HashMapData name: AHashMap uses (globally declared) HashMapData to store hash and element_idx. However, the name makes it sounds like it stores data.
Solution: I've renamed the field to AHashMap::Metadata. This is consistent with naming terminology of hash maps in other libraries.

AHashMap::Metadata is a struct of a union of a struct or uint64_t: This is very confusing.
Solution: I've changed Metadata to be a struct, and whenever both elements are assigned at once, a Metadata instance is used, instead of assigning .data union. I tested performance (get, get missing, insert, insert existing, erase), and found no regressions (if anything, it's 1-2% faster now).

Indexes are inconsistently named: Indexes are named pos, index, idx, kpos, hpos, among others. Further, in AHashMap and HashSet, pos may refer to either key or hash index inconsistently, even within the same function. This makes it extremely difficult to know at a glance which kind of index you're looking at.
Solution: HashMap has only one kind of index. All indices are called idx. AHashMap has elements and metadata, so it now uses metadata_idx and element_idx. HashSet has hashes and keys, so it now uses hash_idx and key_idx. capacity_idx is now consistently named capacity_idx.

Fields are indistinguishable from local variables: For example, AHashMap has local capacity and the field capacity.
Solution: Private fields are prefixed with underscore _ to mark them as such.

capacity is indistinguishable from capacity mask: AHashMap has field capacity, but it's actually capacity - 1.
Solution: I've renamed the field to _capacity_mask.

num_elements is inconsistent with size of other containers: Other collections like Array use size() to refer to the number of elements in the collection. The public APIs of hash maps also use size(), but the internal field is called num_elements.
Solution: num_elements is renamed to size for consistency.

There are other, smaller changes to pull everything together, but I think this covers the big ones.