[hub] Hub is not a SequenceContainer

std::hive is explicitly stated as not meeting even Container requirements,
not to mention SequenceContainer requirements:
https://eel.is/c++draft/hive.overview#6

Its location in clause 23.3 "Sequence containers" makes the situation
unnecessarily confusing.

Container requirements require equality comparison, which std::hive, and
therefore boost::container::hub, does not provide. It is debatable whether
a container should provide it. It is debatable whether std::unordered_map
should provide a quadratic comparison operator only to satisfy the concept.
But it is what it is. But this is the least of the concerns.

std::hive, and therefore boost::container::hub cannot be a
SequenceContainer,because the whole idea of the concept is that the user
controls the order of elements manually via insertions at desired places.
This is reflected in operation

a.insert(p, t)
Effects: Inserts a copy of `t` before `p`.

(https://eel.is/c++draft/sequence.reqmts#26)

std::hive provides an operation with this syntax, but with different
semantics: the value of the iterator will be ignored and the container will
insert the element at an arbitrary location. Users cannot control the
sequence of elements. std::hive is more like std::multiset in this aspect
(modulo the performance benefits). But the specification of std::hive,
although misleading, in the end states clearly that it is not a sequence
container.

The docs for boost::container::hub should make it abundantly clear that it
does not satisfy the requirements of a SequenceContainer.

Regards,
&rzej;
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