Poor hash uniformity and/or a crafted or unlucky input could cause the
bounds of the PSL stats array to be exceeded, which caused memory
corruption (not good!) -- we avoid such cases now by returning an
OutOfMemory error if we're about to insert and there's an item with a
PSL in the last slot.
Previous logic had multiple issues that were hiding in edge cases of
edge cases with the ressurected item handling among other things; the
added assertIntegrity method finds these issues, the primary one being
an edge case where an ID is present in two different buckets.
Added more comments to explain logic in more detail and fixed a couple
little things like always using `+%` when incrementing the probe pos,
and replacing a silent return on an integrity issue that should be
impossible (`table[item.meta.bucket] != id`) with an assert.
This enables these funcs to access memory offsets that may be present in
set items, which is possible since the set itself is in an offset-based
structure.
Prevent bad input from causing repeated OutOfMemory errors by erroring
with NeedsRehash instead when there are unused dead IDs available.
Additionally, properly decrement PSL stats when reviving dead IDs.
