Change BoxIter to avoid trait objects

The introduction of object-safety for coercion to trait objects (see
rust-lang/rfcs#255) affects the previous implementation of BoxIter. It turns
out trait objects are not actually necessary to deal with this situation, so
the implementation was changed to be generic over the type of iterator instead
of the type iterated over.

src/boxiter.rs

@@ -8,27 +8,29 @@
 /// Iterator trait.
 ///
 /// Iterator element type is `A`
-pub struct BoxIter<A> {
+pub struct BoxIter<I> {
     /// The wrapped iterator pointer
-    pub iter: Box<Iterator<A> + 'static>
+    pub iter: Box<I>
 }
 
-impl<A> BoxIter<A>
+impl<A, I> BoxIter<I>
+    where I: Iterator<A>
 {
     /// Create a BoxIter from an iterator value
-    pub fn from_iter<I: 'static + Iterator<A>>(iter: I) -> BoxIter<A>
+    pub fn from_iter(iter: I) -> BoxIter<I>
     {
-        BoxIter::from_box(box iter as Box<Iterator<A> + 'static>)
+        BoxIter::from_box(box iter)
     }
 
     /// Create a BoxIter from an already boxed iterator
-    pub fn from_box(iter: Box<Iterator<A> + 'static>) -> BoxIter<A>
+    pub fn from_box(iter: Box<I>) -> BoxIter<I>
     {
         BoxIter{iter: iter}
     }
 }
 
-impl<A> Iterator<A> for BoxIter<A>
+impl<A, I> Iterator<A> for BoxIter<I>
+    where I: Iterator<A>
 {
     #[inline]
     fn next(&mut self) -> Option<A>