Update converters to take a consistent set of parameters.

As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.

docs/howto/custom-model-fields.txt

@@ -477,7 +477,7 @@ instances::
     class HandField(models.Field):
         # ...
 
-        def from_db_value(self, value, connection, context):
+        def from_db_value(self, value, expression, connection, context):
             if value is None:
                 return value
             return parse_hand(value)