Insufficient archive validation can cause out-of-bounds reads in archives containing hash tables

The archive validator could accept certain malformed relative pointers and
invalid `ArchivedHashTable` states. In particular, the hash table verifier did
not ensure that the number of occupied buckets matched the table's declared
length.

A crafted archive could pass the checks performed by the safe `rkyv::access`
and `rkyv::from_bytes` APIs and then cause an out-of-bounds read in later
validation, lookup, or deserialization. Depending on the input, this could
perform scalar or SIMD reads outside the archive buffer or crash the process.

Version 0.8.17 strengthens archive range validation and rejects hash tables
whose number of occupied buckets does not match their declared length. Users
who process untrusted archives should upgrade to 0.8.17 or later.