Implement a faster sort algorithm

This is a complete rewrite of the standard sort algorithm. The new algorithm
is a simplified variant of TimSort. In summary, the changes are:

* Improved performance, especially on partially sorted inputs.
* Performs less comparisons on both random and partially sorted inputs.
* Decreased the size of temporary memory: the new sort allocates 4x less.

src/libcollections/lib.rs

@@ -47,14 +47,14 @@
 #![feature(placement_in)]
 #![feature(placement_new_protocol)]
 #![feature(shared)]
+#![feature(slice_get_slice)]
 #![feature(slice_patterns)]
 #![feature(specialization)]
 #![feature(staged_api)]
-#![feature(step_by)]
 #![feature(trusted_len)]
 #![feature(unicode)]
 #![feature(unique)]
-#![feature(slice_get_slice)]
+#![feature(untagged_unions)]
 #![cfg_attr(test, feature(rand, test))]
 
 #![no_std]