Add casual string representation of named settings and predicates.

Use 'group.setting' format for named predicates, only display the expression
for anonymous predicates.

meta/cretonne/__init__.py

@@ -35,6 +35,9 @@ class Setting(object):
         self.byte_offset = None
         self.group = SettingGroup.append(self)
 
+    def __str__(self):
+        return '{}.{}'.format(self.group.name, self.name)
+
     def predicate_context(self):
         """
         Return the context where this setting can be evaluated as a (leaf)

meta/cretonne/predicates.py

@@ -66,12 +66,12 @@ class Predicate(object):
         assert self.context, "Incompatible predicate parts"
 
     def __str__(self):
-        s = '{}({})'.format(
+        if self.name:
+            return '{}.{}'.format(self.context.name, self.name)
+        else:
+            return '{}({})'.format(
                     type(self).__name__,
                     ', '.join(map(str, self.parts)))
-        if self.name:
-            s = '{}={}'.format(self.name, s)
-        return s
 
     def predicate_context(self):
         return self.context