MergeInteractions step should do better at single-qubit gate outputs

[Strilanc]

Currently, MergeInteractions is written in a way that assumes MergeRotations will be run afterwards. But I've seen several people not do this. So we should fix that by having MergeInteractions do a better job of optimizing the single-qubit rotations that come out.

[Strilanc]

Obsoleted by #647