Add Extend::{extend_one,extend_reserve}

This adds new optional methods on `Extend`: `extend_one` add a single
element to the collection, and `extend_reserve` pre-allocates space for
the predicted number of incoming elements. These are used in `Iterator`
for `partition` and `unzip` as they shuffle elements one-at-a-time into
their respective collections.

src/libstd/lib.rs

@@ -263,6 +263,7 @@
 #![feature(duration_constants)]
 #![feature(exact_size_is_empty)]
 #![feature(exhaustive_patterns)]
+#![feature(extend_one)]
 #![feature(external_doc)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]