Currently they're not documented, so maybe we can get away with
changing them. Alexey has been threatening to update the hash table
documentation, and even sent me some patches, so now seems like a good
time. Incidentally, there's even a (vestigial) redefinition in
edwin/nntp.scm of HASH-TABLE/MODIFY! with the argument order I prefer.
(The real problem is that we don't have any reasonable way to handle
API and ABI compatibility. Some day, perhaps...)