Add hashset reads

[ero-qt]

Depends on #150.
Part of #149.

Adds get_hash_set_offsets and read_hash_set to both backends' Module, on the dictionary machinery: a slot is a dictionary entry without the key, so resolution and the chunked scan are the same code handed different names.

• Resolution accepts both corlib naming generations (_slots/_buckets/_count/_lastIndex and the m_-prefixed ones). One thing worth writing down: the set's bucket heads are one-based in both generations, unlike the dictionary's split. Nothing reads buckets yet, but whoever adds keyed lookups will want that recorded.
• read_hash_set walks to _lastIndex, the high-water mark, since freed slots sit inside it, and the live tally has to equal _count exactly or the read fails. The value type is the caller's claim, refused past the slot's room, like the dictionary's.