api: implement FusedIterator

`IntoIter` and `FilterEntry<IntoIter, P>` will return `None` once they
returned `None`. Implementing `FusedIterator` allows `Iterator::fuse`
method to be optimized.

PR #139

src/lib.rs

@@ -113,6 +113,7 @@ use std::cmp::{min, Ordering};
 use std::fmt;
 use std::fs::{self, ReadDir};
 use std::io;
+use std::iter;
 use std::path::{Path, PathBuf};
 use std::result;
 use std::vec;
@@ -1001,6 +1002,8 @@ impl IntoIter {
     }
 }
 
+impl iter::FusedIterator for IntoIter {}
+
 impl DirList {
     fn close(&mut self) {
         if let DirList::Opened { .. } = *self {
@@ -1083,6 +1086,11 @@ where
     }
 }
 
+impl<P> iter::FusedIterator for FilterEntry<IntoIter, P> where
+    P: FnMut(&DirEntry) -> bool
+{
+}
+
 impl<P> FilterEntry<IntoIter, P>
 where
     P: FnMut(&DirEntry) -> bool,