Remove duplicated Tensor logic

[crockeea]

We have logic for tensoring prime transforms to prime-power transforms, and for tensoring prime-power transforms, in both Tensor.hs and RTCommon.hs.

In a personal communication, cpeikert also noted:

All the reindexing stuff (extIndices* and baseIndices*) between Zm^* and [n] has duplication. I toyed with the idea of having a special DSL for binary or ternary (0, +-1) matrices that have a single nonzero entry per row or column, which I think is general enough to capture all the two-index transformations without killing efficiency. We could even preserve the ext/baseIndices vectors if desired, but compute them in a nicer way.