Eliminate operator string-typing in `hydroflow_lang`

[MingweiSamuel]

Are these "stringly typed"? Should they be fixed (separately)?

Originally posted by @rohitkulshreshtha in https://github.com/hydro-project/hydroflow/pull/1198#discussion_r1605306725

if let Some(op_inst) = self.flat_graph.node_op_inst(pred_node_id) {
    if "persist" == op_inst.op_constraints.name
        || "state" == op_inst.op_constraints.name
    {
        continue;
    }
}

May want to make a new has_replay field for the above use case

[shadaj]

This will probably be a good project to tackle along with the bounded versus unbounded type system in the new semantics proposal / design.

[MingweiSamuel]

ensure_singleton_referencers_succeed_persist is removed

No toher string-typing exists

DEFER_TICK.name != op_inst.op_constraints.name exists but should be OK, since it uses the name variable instead of a magic string