Merge #376

376: 0.8.1 release r=jswrenn a=danielhenrymantilla

Updated Changelog and `Cargo.toml` for the ~~0.9.0~~ 0.8.1 release, as suggested in #366 (comment)

  - [Rendered](https://github.com/rust-itertools/itertools/blob/a7a1203960451fe2ba21394e5b38981ef8323181/README.rst)

Co-authored-by: Daniel Henry-Mantilla <daniel.henry.mantilla@gmail.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "itertools"
-version = "0.8.0"
+version = "0.8.1"
 
 license = "MIT/Apache-2.0"
 repository = "https://github.com/bluss/rust-itertools"

README.rst

@@ -48,6 +48,97 @@ then it can't be accepted into ``libcore``, and you should propose it for ``iter
 Recent Changes
 --------------
 
+- 0.8.1
+
+  - Added a `.exactly_one() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.exactly_one>`_
+    iterator method that, on success, extracts the single value of an
+    iterator
+    ; by `@Xaeroxe <https://github.com/Xaeroxe>`_
+
+  - Added combinatory iterator adaptors:
+
+    - `.permutations(k) <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.permutations>`_:
+
+      ``[0, 1, 2].iter().permutations(2)`` yields
+
+      .. code:: rust
+
+        [
+            vec![0, 1],
+            vec![0, 2],
+            vec![1, 0],
+            vec![1, 2],
+            vec![2, 0],
+            vec![2, 1],
+        ]
+
+      ; by `@tobz1000 <https://github.com/tobz1000>`_
+
+    - `.combinations_with_replacement(k) <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.combinations_with_replacement>`_:
+
+      ``[0, 1, 2].iter().combinations_with_replacement(2)`` yields
+
+      .. code:: rust
+
+        [
+            vec![0, 0],
+            vec![0, 1],
+            vec![0, 2],
+            vec![1, 1],
+            vec![1, 2],
+            vec![2, 2],
+        ]
+
+      ; by `@tommilligan <https://github.com/tommilligan>`_
+
+    - For reference, these methods join the already existing
+      `.combinations(k) <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.combinations>`_:
+
+      ``[0, 1, 2].iter().combinations(2)`` yields
+
+      .. code:: rust
+
+        [
+            vec![0, 1],
+            vec![0, 2],
+            vec![1, 2],
+        ]
+
+  - Improved the performance of `.fold() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.fold>`_-based internal iteration for the
+    `.intersperse() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.intersperse>`_ iterator
+    ; by `@jswrenn <https://github.com/jswrenn>`_
+
+  - Added
+    `.dedup_by() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.dedup_by>`_,
+    `.merge_by() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.merge_by>`_
+    and `.kmerge_by() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.kmerge_by>`_
+    adaptors that work like
+    `.dedup() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.dedup>`_,
+    `.merge() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.merge>`_ and
+    `.kmerge() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.kmerge>`_,
+    but taking an additional custom comparison closure parameter.
+    ; by `@phimuemue <https://github.com/phimuemue>`_
+
+  - Improved the performance of `.all_equal() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.all_equal>`_
+    ; by `@fyrchik <https://github.com/fyrchik>`_
+
+  - Loosened the bounds on `.partition_map() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.partition_map>`_
+    to take just a ``FnMut`` closure rather than a ``Fn`` closure, and made its
+    implementation use internal iteration for better performance
+    ; by `@danielhenrymantilla <https://github.com/danielhenrymantilla>`_
+
+  - Added convenience methods to
+    `EitherOrBoth <https://docs.rs/itertools/0.8.1/itertools/enum.EitherOrBoth.html>`_ elements yielded from the
+    `.zip_longest() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.zip_longest>`_ iterator adaptor
+    ; by `@Avi-D-coder <https://github.com/Avi-D-coder>`_
+
+  - Added `.sum1() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.sum1>`_
+    and `.product1() <https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.product1>`_
+    iterator methods that respectively try to return the sum and the product of
+    the elements of an iterator **when it is not empty**, otherwise they return
+    ``None``
+    ; by `@Emerentius <https://github.com/Emerentius>`_
+
 - 0.8.0
 
   - Added new adaptor ``.map_into()`` for conversions using ``Into`` by @vorner