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.

Objects/floatobject.c

@@ -302,7 +302,7 @@ float_repr(PyFloatObject *v)
 {
 	char buf[100];
 	format_float(buf, sizeof(buf), v, PREC_REPR);
-	return PyString_FromString(buf);
+	return PyUnicode_FromString(buf);
 }
 
 static PyObject *