Add an access abstraction (only array tests work, yet)

The access abstraction will be the new way of accessing builtin objects. This way it will be easier to move that to another process

jedi/evaluate/helpers.py

@@ -183,7 +183,7 @@ def is_compiled(context):
 
 
 def is_string(context):
-    return is_compiled(context) and isinstance(context.obj, (str, unicode))
+    return is_compiled(context) and isinstance(context.get_safe_value(default=None), (str, unicode))
 
 
 def is_literal(context):
@@ -191,4 +191,4 @@ def is_literal(context):
 
 
 def is_number(context):
-    return is_compiled(context) and isinstance(context.obj, (int, float))
+    return is_compiled(context) and isinstance(context.get_safe_value(default=None), (int, float))