Add hashset reads

[ero-qt]

Part of #149.

Adds get_hash_set_offsets and read_hash_set to both backends' Module, built 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. The set's bucket heads are one-based in both generations, unlike the dictionary's. Nothing reads buckets yet, but whoever adds keyed lookups will want that written down.

read_hash_set walks to _lastIndex, the high-water mark, since freed slots sit inside it. 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.

cargo test --all-features runs the suite. The new tests cover both naming generations, live values on both runtimes, and unbalanced tallies.

[CryZe]

I’m waiting for this PR to be rebased onto the current master before continuing the review. It currently still contains the PR #150 commits in its history and GitHub reports conflicts. Please let me know once it has been rebased.

[ero-qt]

Rebased. It's on master now with just its own commit, and I folded the same class hierarchy check and the empty case in for hash sets while I was at it.

[CryZe]

Reviewed the public API, implementation, and tests. Also validated the IL2CPP path against Lunistice with a live HashSet, including the canonical empty/unallocated representation; offset and slot layout resolution and the read all passed.