Make it possible for users to infer annotations/defaults

Fixes #1039

jedi/api/classes.py

@@ -618,7 +618,7 @@ class Signature(Definition):
 
     @property
     def params(self):
-        return [Definition(self._evaluator, n)
+        return [ParamDefinition(self._evaluator, n)
                 for n in self._signature.get_param_names(resolve_stars=True)]
 
 
@@ -660,6 +660,14 @@ class CallSignature(Signature):
         )
 
 
+class ParamDefinition(Definition):
+    def infer_default(self):
+        return [Definition(self._evaluator, d.name) for d in self._name.infer_default()]
+
+    def infer_annotation(self):
+        return [Definition(self._evaluator, d.name) for d in self._name.infer_annotation()]
+
+
 def _format_signatures(context):
     return '\n'.join(
         signature.to_string()