Implement ExactSizeIterator for remaining core Iterators where applicable.

Specifically:
 * Peekable
 * ByRef
 * Skip
 * Take
 * Fuse

src/libcore/iter.rs

@@ -1087,6 +1087,12 @@ impl<'a, I> DoubleEndedIterator for ByRef<'a, I> where I: 'a + DoubleEndedIterat
     fn next_back(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next_back() }
 }
 
+#[stable]
+impl<'a, I> ExactSizeIterator for ByRef<'a, I> where I: 'a + ExactSizeIterator {
+    #[inline]
+    fn len(&self) -> uint { self.iter.len() }
+}
+
 /// A trait for iterators over elements which can be added together
 #[unstable = "needs to be re-evaluated as part of numerics reform"]
 pub trait AdditiveIterator<A> {
@@ -1790,6 +1796,16 @@ fn size_hint(&self) -> (uint, Option<uint>) {
     }
 }
 
+#[stable]
+impl<T, I> ExactSizeIterator for Peekable<T, I> where I: ExactSizeIterator<Item = T> {
+    #[inline]
+    fn len(&self) -> usize {
+        // This is guarenteed to not overflow because `len()` must have been able to return a valid
+        // value before we peeked.
+        self.iter.len() + if self.peeked.is_some() { 1 } else { 0 }
+    }
+}
+
 #[stable]
 impl<T, I> Peekable<T, I> where I: Iterator<Item=T> {
     /// Return a reference to the next element of the iterator with out advancing it,
@@ -1982,6 +1998,12 @@ fn idx(&mut self, index: uint) -> Option<<I as Iterator>::Item> {
     }
 }
 
+#[stable]
+impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {
+    #[inline]
+    fn len(&self) -> uint { self.iter.len().saturating_sub(self.n) }
+}
+
 /// An iterator that only iterates over the first `n` iterations of `iter`.
 #[derive(Clone)]
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
@@ -2037,6 +2059,12 @@ fn idx(&mut self, index: uint) -> Option<<I as Iterator>::Item> {
     }
 }
 
+#[stable]
+impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {
+    #[inline]
+    fn len(&self) -> uint { cmp::min(self.iter.len(), self.n) }
+}
+
 
 /// An iterator to maintain state while iterating another iterator
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
@@ -2246,6 +2274,12 @@ fn idx(&mut self, index: uint) -> Option<<I as Iterator>::Item> {
     }
 }
 
+#[stable]
+impl<I> ExactSizeIterator for Fuse<I> where I: ExactSizeIterator {
+    #[inline]
+    fn len(&self) -> uint { self.iter.len() }
+}
+
 impl<I> Fuse<I> {
     /// Resets the fuse such that the next call to .next() or .next_back() will
     /// call the underlying iterator again even if it previously returned None.