Refactor stdlib PartialObject

Merges PartialObject and PartialMethodObject. Also adds more tests.
Some parts are still WIP, see: #1522.

Fixes #1519

test/completion/stdlib.py

@@ -169,6 +169,11 @@ X().a('')[0]
 #? str()
 X().a('')[1]
 
+#? int()
+X.a('')[0]
+#? str()
+X.a('')[1]
+
 #? int()
 X.a(X(), '')[0]
 #? str()
@@ -180,6 +185,12 @@ tup[0]
 #? float()
 tup[1]
 
+tup = X.kw(1)
+#? int()
+tup[0]
+#? float()
+tup[1]
+
 tup = X.kw(X(), 1)
 #? int()
 tup[0]