[PATCH packages] Implement assoc-string

Jerry James quotes some unnamed Emacs bagbuilder:

 > +(defun assoc-string (key list &optional case-fold)
 > +  "Like `assoc' but specifically for strings (and symbols).
 > +
 > +This returns the first element of LIST whose car matches the string or
 > +symbol KEY, or NIL if no match exists.  When performing the comparison,
 > +symbols are first converted to strings.  If the optional arg CASE-FOLD
 > +is non-nil, case is ignored.
 > +
 > +Unlike `assoc', KEY can also match an entry in LIST consisting of a
 > +single string, rather than a cons cell whose car is a string."

And RMS calls the impossibility of parsing #r"?\" backwards is a wart!

Maintaining compatibility with Emacs is worse than having a
schizophrenic spouse!