Implement some trivial size_hints for various iterators

This also implements ExactSizeIterator where applicable.

Addresses most of the Iterator traits mentioned in #23708.

src/libsyntax_ext/format_foreign.rs

@@ -272,6 +272,11 @@ pub mod printf {
             self.s = tail;
             Some(sub)
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            // Substitutions are at least 2 characters long.
+            (0, Some(self.s.len() / 2))
+        }
     }
 
     enum State {
@@ -782,6 +787,10 @@ pub mod shell {
                 None => None,
             }
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (0, Some(self.s.len()))
+        }
     }
 
     /// Parse the next substitution from the input string.