assert_matrix_eq! improvements (tracking issue)

[Andreas Borgen Longva]

In this issue, I intend to maintain a list of suggestions for improvements to the assert_matrix_eq! and assert_vector_eq! macros. As more contributors get a chance to use it, I would appreciate input on missing features that would make working with it easier. Actual bugs in these macros should still be reported in their own issue.

Potential usability improvements:

• We may want to consider implementing an assert_scalar_eq! for scalar comparisons, leveraging the same comparators. Currently we have no facility for this in rulinalg, and it's a situation that comes up a lot in tests (such as testing that the determinant is almost exactly equal to an expected value).
• Print both matrices when the dimension is reasonably small. Currently it's still a little difficult to get a feel for exactly what is wrong in some cases involving small matrices.

Other improvements:

• Consider letting users provide their own comparators (low-priority unless we get requests for this).
• Avoid Copy trait bounds and avoid storing copied elements instead of references.