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/libstd/lib.rs

@@ -232,6 +232,7 @@
 #![feature(float_from_str_radix)]
 #![feature(fn_traits)]
 #![feature(fnbox)]
+#![feature(fused)]
 #![feature(hashmap_hasher)]
 #![feature(heap_api)]
 #![feature(inclusive_range)]