`Spring.SetAlly` being one-way is probably a bad default

Bill of materials:
* probably best to wait until #1656 gets resolved somehow, to avoid introducing new interface that will just get renamed again soon

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See title, one-way allying is rarely desirable so the salient default should probably be mutual allying. Perhaps there should be a new `SetOneWayAlly` (or as appropriate if there was a rename due to #1656).

Start by reviewing the Spring.SetAlly interface and the unresolved dependency on #1656. Determine the final interface naming there, then define mutual allying as the default while preserving an explicitly one-way option; the issue is done when that API behavior and naming are settled and implemented.

Written by the indexing model from the issue text.