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/liballoc/string.rs

@@ -2254,5 +2254,5 @@ impl<'a> DoubleEndedIterator for Drain<'a> {
     }
 }
 
-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]
 impl<'a> FusedIterator for Drain<'a> {}