Add a ContextSet.

This is not bug free yet, but it's going to be a good abstraction for a lot of small things.

jedi/evaluate/param.py

@@ -237,7 +237,7 @@ class ExecutedParam(object):
         pep0484_hints = pep0484.infer_param(self._execution_context, self._param_node)
         doc_params = docstrings.infer_param(self._execution_context, self._param_node)
         if pep0484_hints or doc_params:
-            return list(set(pep0484_hints) | set(doc_params))
+            return pep0484_hints | doc_params
 
         return self._lazy_context.infer()