scripts: csv.py: Tweaked foldchecking to check that folds match

I mean, what would you expect this to do?

  max(a) + sum(b)

Whatever your answer is, it's wrong (the way csv.py works, we always
compute folds after expr evaluation). The best option is to error,
matching the behavior of mismatched types.

scripts/csv.py

@@ -538,7 +538,10 @@ class CsvExpr:
             return t
 
         def fold(self, types={}):
-            return self.a.fold(types)
+            f = self.a.fold(types)
+            if not all(f == v.fold(types) for v in it.islice(self, 1, None)):
+                raise CsvExpr.Error("mismatched folds? %r" % self)
+            return f
 
         def eval(self, fields={}, state=None):
             return self.a.eval(fields, state)