Cross-reference the 'Fundamentals' module with the 'Idiomatic Rust' module

Author: LukeMathWalker

src/idiomatic/leveraging-the-type-system/newtype-pattern.md

@@ -34,6 +34,10 @@ assert_ne!(UserId(1), UserId(2));
 
 <details>
 
+- Students should have encountered the newtype pattern in the "Fundamentals"
+  course, when they learned about
+  [tuple structs](../../user-defined-types/tuple-structs.md).
+
 - Run the example to show students the error message from the compiler.
 
 - Modify the example to use a typealias instead of a newtype, such as

src/user-defined-types/tuple-structs.md

@@ -43,6 +43,8 @@ fn main() {
   - The value passed some validation when it was created, so you no longer have
     to validate it again at every use: `PhoneNumber(String)` or
     `OddNumber(u32)`.
+- The newtype pattern is covered extensively in the
+  ["Idiomatic Rust" module](../idiomatic/leveraging-the-type-system/newtype-pattern.md).
 - Demonstrate how to add a `f64` value to a `Newtons` type by accessing the
   single field in the newtype.
   - Rust generally doesn’t like inexplicit things, like automatic unwrapping or