msg: Walk the fields in another way, e.g. used when decoding

ISUPTests.st

@@ -33,6 +33,10 @@ TestCase subclass: ISUPGeneratedTest [
                 each create].
             struct optionals do: [:each |
                 each create].
+
+            "same thing once more"
+            struct fieldsDo: [:type :class |
+                class create].
         ].
     ]
 ]

MessageStructure.st

@@ -63,6 +63,10 @@ Object subclass: MSGStructure [
         ^ fields ifNil: [fields := OrderedCollection new]
     ]
 
+    fieldsDo: aBlock [
+        ^ self fields do: [:each | aBlock value: each first value: each second]
+    ]
+
     filter: aFilter [
         | lst |
         lst := OrderedCollection new.