STRING coercion

[JONL at MIT-MC (Jon L White)]

The maclisp string package now has STRING accepting a list as
argument, in which case it does something like
  (APPLY 'STRING-APPEND (MAPCAR 'STRING <list-argument>))
In particular, this means that STRING will work as a general
compress-list-of-characters/fixnums-into-string.   Would the 
LISPM be capable of extending the definition of STRING too?
(N.B.: the phrase "something like" means that it is only one
level deep - kludged for speed - thus (STRING '( (65. 66.) 67.))
loses, whereas (STRING '(ABC "def" 71.)) ==> "ABCdefG")
Note also the addition of helpfun functions STRING-EXPLODEN
and STRING-PRIN1, for the maclisp-without-nilaid world.