new version of excl-low

[Gregor.pa@Xerox.COM]

This message is only important for people using PCL in ExCL (Franz
Common Lisp).

There is a new version of excl-low.lisp on parcvax.xerox.com.  This
fixes a bug which caused slot lookup caching to always miss.  Anyone
using PCL in ExCL should get a copy of this file and then use
(compile-pcl) to recompile their pcl, and then recompile their files
that use PCL.

This should result in a significant speed improvement for many programs.
Some cases of method lookup will be 5 times faster, compiled slot-access
will be much faster.  Its hard to quantify exactly, but it should
definitely be noticeable.