Add embeddings for siProp into uPred and monPred

This adds an embedding of siProp into uPred, since I didn't find one in the code base. It also adds a "transitive" embedding, which constructs an embedding of PROP1 into PROP3, assuming embeddings of PROP1 into PROP2 and PROP2 into PROP3.

The embedding from siProp into monPred is then constructed from these embeddings and the existing embedding of uPred into monPred.

This is a contribution (on behalf) of BedRock Systems.