Add iterop types for const-generic-dimensional types

[saolof]

Since the const generics mvp was just stabilized, a large number of const generics linear algebra packages are hitting stable at roughly the same time. A standard to make them all compatible with each other would be great to have.

So a possible set of types would be n-dimensional points, n dimensional vectors, m x n dimensional row matrices, and m x n dimensional column matrices.

Of course, there is also the possibility of taking the existing types with hardcoded dimensions and turning them into aliases for the generic-dimension types specialized at various dimensions, though that would be a separate issue.