Added a Lookahead iterator.

This adds a Lookahead iterator so that while parsing it is easier
to peek ahead however much the parser needs. Basic parsers may not
need any, but a lot of parsers have two token lookahead. I've even
seen some with three.

tests/iter.rs

@@ -0,0 +1,43 @@
+use rune::*;
+
+#[test]
+fn peek_works()
+{
+   let mut it = vec![1, 2, 3].into_iter().lookahead();
+   assert_eq!(it.peek(0), Some(&1));
+   assert_eq!(it.peek(1), Some(&2));
+   assert_eq!(it.peek(2), Some(&3));
+   assert_eq!(it.peek(3), None);
+}
+
+#[test]
+fn peek_mut_modifies_item()
+{
+   let mut it = vec![10, 20, 30].into_iter().lookahead();
+   if let Some(x) = it.peek_mut(1)
+   {
+      *x += 100;
+   }
+   assert_eq!(it.next(), Some(10));
+   assert_eq!(it.next(), Some(120));
+}
+
+#[test]
+fn iterates_correctly()
+{
+   let mut it = vec![1, 2, 3].into_iter().lookahead();
+   assert_eq!(it.next(), Some(1));
+   assert_eq!(it.next(), Some(2));
+   assert_eq!(it.next(), Some(3));
+   assert_eq!(it.next(), None);
+}
+
+#[test]
+fn size_hint_accounts_for_buffer()
+{
+   let mut it = vec![1, 2, 3, 4].into_iter().lookahead();
+   it.peek(2);
+   let (low, high) = it.size_hint();
+   assert!(low >= 4);
+   assert_eq!(high, Some(4));
+}