Add a predicate_leafs() method.

This collects all of the leaf predicates that go into a compound predicate.
Current leaf predicates are:

- Settings for ISA predicates, and
- FieldPredicates for instruction predicates.

meta/cretonne/__init__.py

@@ -41,6 +41,9 @@ class Setting(object):
         """
         return self.group
 
+    def predicate_leafs(self, leafs):
+        leafs.add(self)
+
 
 class BoolSetting(Setting):
     """