GeezerVison

CC @michelp who requested this.

When n=0, I decided to create a complete diagonal matrix using the identity of the binaryop if the identity exists (and raise if it doesn't).

We want the following equality to hold:

I = A.power(0, semiring)
semiring(I @ A @ I).isequal(A)

We could try to make the diagonal more sparse to better support hypersparse matrices, but this would need to determine both nonempty rows and nonempty columns, because I needs to be able to be the left-identity and the right-identity. Creating the complete diagonal matrix is much simpler.

CC @jim22k. When we added Matrix.power in #483, I believe we discussed n=0, and we decided to punt until it was needed. As we discussed, n=0 is a little awkward b/c the identity value isn't always available (since it ought to come from the binaryop--not the monoid--of the semiring), and we weren't sure whether we should try to return a sparse diagonal or if this is what the user would want and expect. I think the behavior in this PR is the best approach.