core: Stabilize FusedIterator

FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).

The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.

src/libstd/lib.rs

@@ -266,7 +266,6 @@
 #![feature(float_from_str_radix)]
 #![feature(fn_traits)]
 #![feature(fnbox)]
-#![feature(fused)]
 #![feature(generic_param_attrs)]
 #![feature(hashmap_hasher)]
 #![feature(heap_api)]