Comparison doc for semantics discussion

I took Bruce's and Deyan's table of examples and added my two proposals to
the table so that we can compare them side by side. I managed to get most
of the way through the examples, but ran out of time to finish. The
examples with derivatives and integrals at the end are problematic in all
notations.

The comparison doc is at
https://mathml-refresh.github.io/mathml/docs/comparison (you need a wide
screen to see all the table entries in a row). Discussing this will be the
focus for most of the July 2 call.

    Neil