better definitions of SML90 features

--- a/src/Pure/ML-Systems/smlnj.ML	Thu Dec 21 19:19:18 2000 +0100
+++ b/src/Pure/ML-Systems/smlnj.ML	Fri Dec 22 13:53:28 2000 +0100
@@ -9,10 +9,10 @@
 
 (* restore old-style character / string functions *)
 
-fun ord s = Char.ord (String.sub (s, 0));
-val chr = str o Char.chr;
-val explode = (map str) o String.explode;
-val implode = String.concat;
+val ord     = SML90.ord;
+val chr     = SML90.chr;
+val explode = SML90.explode;
+val implode = SML90.implode;
 
 
 (* New Jersey ML parameters *)