Add a FusedIterator trait.

This trait can be used to avoid the overhead of a fuse wrapper when an iterator
is already well-behaved.

Conforming to: RFC 1581
Closes: #35602

src/liballoc/lib.rs

@@ -91,7 +91,7 @@
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(unsize)]
 
-#![cfg_attr(not(test), feature(raw, fn_traits, placement_new_protocol))]
+#![cfg_attr(not(test), feature(fused, raw, fn_traits, placement_new_protocol))]
 #![cfg_attr(test, feature(test, box_heap))]
 
 // Allow testing this library