Deprecate/refactor old stateful/replaying operators

[MingweiSamuel]

• [ ] replace join with state_join which can use keyed datastructures from state() op #969, bypass #564
• [ ] remove persist() (or replace with state()), replace [lattice_]reduce/fold with state() versions #925
• [ ] mutable state with state_mut() operator, with extra delete input #785
• [ ] Deprecate message for others

[jhellerstein]

Replay on joins/bimorphisms is now working correctly by insisting on persisting the inputs (which is required semantically for bimorphism).

Remaining work plan is around references:

1. Introduce an n-way cross operator to gather refs. Give it a meaningful name (not cross-join; maybe "tuple" or "gatherRefs" or something)
2. Make sure/rewrite code so that output of the n-way cross is reflected in argument of its consumer.
3. Check that replay of refs through the cross operator works as expected.

[jhellerstein]

We could do this now, but it would be better to gather more experience, and perhaps wait for more helpers in Hydroflow+ before we remove the more ergonomic existing "convenience" operators.

[MingweiSamuel]

Low priority of 0.9 release - hoping to drive from hydroflow+ use cases, and filter downward what works/is needed

And look at Shadaj's Flo paper