This check are from initial ordered hash commit when
first entry were created with entry->fore = entry->back = entry

…ot used

It is not used in hash.c since 2005-03-04

Since hash use st_foreach_check for iterations, such checks are needless

It allows order of entries to be stable, so that we could
iterate over entries instead of FIND_ENTRY.
It will allow us to introduce safe mode for st_table.

…deleted

Embed logic of safe mode and delete mark inside of st_table, so that Hash
could be simplified and st_delete_safe removed at all.

ST_CHECK were always returned instead of ST_CONTINUE inside
iterator for st_foreach_check, and st_foreach is not supposed
for handling ST_CHECK. So that, unify ST_CONTINUE with ST_CHECK.

There is one natural use case for Hash : unique queue.
One of natural place to use it - lib/thread.rb for use instead of
array in ConditionVariable @waiters.
So lets Hash#shift be as fast as possible?