This will allow us to use these traits for other hash-based containers
(like Map). This commit also adds a special case for negative zero
values, because while the equality check used same_value_zero which is
negative/positive zero insensitive, the hash was not.
This makes sure that is<Set> checks done on the Set prototype instead of
on Set instances return false, thereby emulating the behaviour of the
RequireInternalSlot abstract operation.
