add tests; refactor exercise links

exercises/primitive_types/primitive_types4.rs

@@ -39,11 +39,11 @@ fn main() {
 
 
 // Take a look at the Understanding Ownership -> Slices -> Other Slices section of the book:
-// https://doc.rust-lang.org/stable/book/second-edition/ch04-03-slices.html#other-slices
+// https://doc.rust-lang.org/book/ch04-03-slices.html
 // and use the starting and ending indices of the items in the Array
 // that you want to end up in the slice.
 
 // If you're curious why the right hand of the `==` comparison does not
 // have an ampersand for a reference since the left hand side is a
 // reference, take a look at the Deref coercions section of the book:
-// https://doc.rust-lang.org/stable/book/second-edition/ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods
+// https://doc.rust-lang.org/book/ch15-02-deref.html

exercises/primitive_types/primitive_types5.rs

@@ -39,7 +39,7 @@ fn main() {
 
 
 // Take a look at the Data Types -> The Tuple Type section of the book:
-// https://doc.rust-lang.org/stable/book/second-edition/ch03-02-data-types.html#the-tuple-type
+// https://doc.rust-lang.org/stable/book/ch03-02-data-types.html#the-tuple-type
 // Particularly the part about destructuring (second to last example in the section). 
 // You'll need to make a pattern to bind `name` and `age` to the appropriate parts
 // of the tuple. You can do it!!

exercises/primitive_types/primitive_types6.rs

@@ -41,5 +41,5 @@ fn main() {
 // While you could use a destructuring `let` for the tuple here, try 
 // indexing into it instead, as explained in the last example of the 
 // Data Types -> The Tuple Type section of the book:
-// https://doc.rust-lang.org/stable/book/second-edition/ch03-02-data-types.html#the-tuple-type
+// https://doc.rust-lang.org/stable/book/ch03-02-data-types.html#the-tuple-type
 // Now you have another tool in your toolbox!