Add functions PyUnicode_Append() and PyUnicode_AppendAndDel() that mirror
PyString_Concat() and PyString_ConcatAndDel() (the name PyUnicode_Concat()
was already taken).

Change PyObject_Repr() to always return a unicode object.

Update all repr implementations to return unicode objects.

Add a function PyObject_ReprStr8() that calls PyObject_Repr() and converts
the result to an 8bit string.

Use PyObject_ReprStr8() where using PyObject_Repr() can't be done
straightforward.
diff --git a/Objects/weakrefobject.c b/Objects/weakrefobject.c
index 7b5b561..d30b90f 100644
--- a/Objects/weakrefobject.c
+++ b/Objects/weakrefobject.c
@@ -177,7 +177,7 @@
 		      name);
 	Py_XDECREF(nameobj);
     }
-    return PyString_FromString(buffer);
+    return PyUnicode_FromString(buffer);
 }
 
 /* Weak references only support equality, not ordering. Two weak references
@@ -451,7 +451,7 @@
 		  "<weakproxy at %p to %.100s at %p>", proxy,
 		  PyWeakref_GET_OBJECT(proxy)->ob_type->tp_name,
 		  PyWeakref_GET_OBJECT(proxy));
-    return PyString_FromString(buf);
+    return PyUnicode_FromString(buf);
 }