ZVAL library

[Eugene C. Ciccarelli <ECC at MIT-AI>]

I recently made a few changes to the ZVAL library.  For a
description of these and future changes, you can look at the top
of the source file.  If people want, I could make up a mailing
list of users, though I doubt there are many, and send out brief
announcements.  Probably best to avoid mailing them (after this)
to INFO-LISPM.