Rename "side-effects" to be "incidentals"

[wwitzel]

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Deepak has always objected to "side-effects" as they have a negative connotation in formal methods. I do think "incidentals" is an appropriate name. Instead of a "side_effects" Expression method, we should use "_yield_incidentals" with an underscore to indicate that this is not normally called externally. It is called in Judgment.derive_side_effects which will become Judgment.derive_incidentals.