Final version of diagxy

The final version is available as a diagxy.zip from ftp.math.mcgill.ca by anonymous ftp in pub/barr/. This includes the diagxy.tex and diaxydoc.tex. The former differs from the third version only in trivial ways (some extra \ignorespaces were added to try to prevent extraneous spaces from causing problems). The opening pages of the documentation are different, but the examples are all the same.