Remove Self: Sized bound on Iterator::nth.

This changes the signature of nth but can't break any existing code
because:

  1. nth can currently only be overridden on Sized structs.
  2. Extra `Self: Sized` constraints on methods in impls on Sized
     structs are no-ops.

Author: Stebalien

src/libcore/iter.rs

@@ -160,7 +160,7 @@ 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;