core: Remove Self: Sized from Iterator::nth

It is an unnecessary restriction; nth neither needs self to be sized
nor needs to be exempted from the trait object.

It increases the utility of the nth method, because type specific
implementations are available through `&mut I` or through an iterator
trait object.

It is a backwards compatible change due to the special cases of the
`where Self: Sized` bound; it was already optional to include this bound
in `Iterator` implementations.

src/liballoc/boxed.rs

@@ -524,6 +524,9 @@ impl<I: Iterator + ?Sized> Iterator for Box<I> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         (**self).size_hint()
     }
+    fn nth(&mut self, n: usize) -> Option<I::Item> {
+        (**self).nth(n)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {

src/libcore/iter/iterator.rs

@@ -247,7 +247,7 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn nth(&mut self, mut n: usize) -> Option<Self::Item> where Self: Sized {
+    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
         for x in self {
             if n == 0 { return Some(x) }
             n -= 1;
@@ -2179,4 +2179,7 @@ impl<'a, I: Iterator + ?Sized> Iterator for &'a mut I {
     type Item = I::Item;
     fn next(&mut self) -> Option<I::Item> { (**self).next() }
     fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        (**self).nth(n)
+    }
 }