new spreadsheet module

[straz (Steve Strassmann)]

Thanks to Chris DiGiano for contributing this. I've added it to
cambridge.apple.com:/pub/mcl/contrib/ as sheet.lisp.hqx

I've also updated the README file in that directory to 
list, if not describe, everything currently there.

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From: digi@cs.colorado.edu (Chris DiGiano)
Subject: MCL Contribution

This general sheet module is for creating selectable tables of
arbitrary views. Some applications are: spreadsheets, mixed-
text and graphics diagrams, tool palettes, or cellular-
automata simulations.