Update unstable library feature list by Travis CI build 1578

Author: Travis CI

unstable_library_feature.txt

@@ -207,3 +207,4 @@ btree_retain                        79025    2020-11-13 10:23:50 -0800  2020-11-
 nonzero_leading_trailing_zeros      79143    2020-11-17 19:29:15 +0100  2020-11-17 19:29:15 +0100
 array_value_iter_slice              65798    2020-11-19 08:28:49 +0100  2020-11-19 08:28:49 +0100
 slice_fill_with                     79221    2020-11-20 01:42:43 +0100  2020-11-20 01:42:43 +0100
+peekable_peek_mut                   78302    2020-11-23 23:52:19 +0100  2020-11-23 23:52:19 +0100