Add a utility for iterating over all topological orders (#6801)

Use an extension of Kahn's algorithm for finding topological orders that
iteratively makes every possible choice at every step to find all the
topological orders. The order being constructed and the set of possible
choices are managed in-place in the same buffer, so the algorithm takes
linear time and space plus amortized constant time per generated order.

This will be used in an upcoming type optimization.

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