Fix HT flags copying wrt iterator count

Fix HT flags copying wrt iterator count

HT_FLAGS() includes the full flag word, including the iterator
count. When we're fully reassigning it, we need to make sure that
we either really do want to copy the iterator count (as in some
cases in array.c) or we need to mask only the actual flag byte.

Add an assert to hash_iterators_del() to make sure the iterator
count is non-zero (which is how I ran into this) and make sure that
the iterator count is correctly preserved during array splicing.