Add some nominal types for representing algebra elements

[mcy]

This is a WIP, since I expect it to tear up a lot of function signatures in algebra, sseq, and ext. I'm pushing it now to get feedback on the design.

[mcy]

@dalcde Feedback appreciated

[dalcde]

Some minor comments: 1. This is very much a matter of style but I'd prefer if we made the degree/index fields of BasisElem public instead of having getter/setters. 2. I would expect to use BasisElem etc. for modules as well as algebras. Is algebras the best place to put it? 3. Do we use the partial ordering on BasisElem etc. anywhere? 4. I'm pretty sure this is true but we should make sure the branches are optimized away when we apply `multiply` to `BasisElem` and `VectorElem`. Otherwise I like this ergonomic improvement a lot.

[mcy]

Just an update; I've been super hosed the last weeks and have been punting on this since I haven't been approaching this a great way. I think I'll take a look at redoing this by adding a separate, parallel product trait function that uses the new element types (and is just a thin wrapper) and gradually switch everything over to using those.