[Ecls-list] update-instance-for-redefined-class

[Faré]

ECL seems to erroneously provide update-instance-for-redefined-class
with an alist rather than a plist for the values of deleted slots.
The CLHS says plist and uses getf in its examples;
other implementations seem to use a plist indeed.
This was caught by my asdf upgrade tests.
I tried 12.7.1 (git:ec1fe8781a5f137284cc6c0d0cfa6b0cc9b9b291)
and 12.12.1 (git:ebafa5f2752793de6307a49e9255583acf5322dc).

—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org
Innovation is the matter with which civilization is built.
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                — Faré