Replace unimplemented with todo (google#2594)

Author: randomPoison

src/iterators/exercise.md

@@ -13,7 +13,7 @@ return value.
 
 ```rust
 {{#include exercise.rs:offset_differences}}
-    unimplemented!()
+    todo!()
 }
 
 {{#include exercise.rs:unit-tests}}

src/tuples-and-arrays/exercise.md

@@ -31,7 +31,7 @@ This function only operates on 3x3 matrices.
 #![allow(unused_variables, dead_code)]
 
 {{#include exercise.rs:transpose}}
-    unimplemented!()
+    todo!()
 }
 
 {{#include exercise.rs:tests}}

src/unsafe-rust/exercise.md

@@ -58,17 +58,17 @@ functions and methods:
 {{#include exercise.rs:ffi}}
 
 {{#include exercise.rs:DirectoryIterator}}
-        unimplemented!()
+        todo!()
     }
 }
 
 {{#include exercise.rs:Iterator}}
-        unimplemented!()
+        todo!()
     }
 }
 
 {{#include exercise.rs:Drop}}
-        unimplemented!()
+        todo!()
     }
 }