[PATCH packages] Implement assoc-string

+(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."