work on MATRIX...END MATRIX

[Ben Pfaff]

While I was working on the macro processor, I noticed that many of the
uses of the macro processor I found around the Internet also used
MATRIX...END MATRIX, so I decided to work on that next.

I've made significant progress. I now have implemented the most
important parts of the matrix language. Over the coming weeks, I'll
work on the rest.

If anyone has examples of the use of the matrix language, please send
them along so I can make sure that they're supported.