Add string conversions for predicates and encodings.

This is just used for printing better comments in generated code.

meta/cretonne/__init__.py

@@ -1003,6 +1003,9 @@ class EncRecipe(object):
         if instp:
             assert instp.predicate_context() == format
 
+    def __str__(self):
+        return self.name
+
 
 class Encoding(object):
     """
@@ -1034,6 +1037,9 @@ class Encoding(object):
         self.instp = And.combine(recipe.instp, instp)
         self.isap = And.combine(recipe.isap, instp)
 
+    def __str__(self):
+        return '[{}/{:02x}]'.format(self.recipe, self.encbits)
+
     def ctrl_typevar(self):
         """
         Get the controlling type variable for this encoding or `None`.