Generalize circuit product algorithm to support products of compatible circuits with different structures

[loreloc]

That is, cover the missing cases (see #TODOs in the symbolic product function):

• product between an input layer and either a sum or product layer
• product between a sum layer and a product layer
• product between sum layers of different arity

[arranger1044]

We should keep the overlapping scopes case around (perhaps in a different issue?).