Add module exercises from jdm! 🎉

Fixes #9.

modules/modules1.rs

@@ -0,0 +1,42 @@
+// Make me compile! Scroll down for hints :)
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+mod sausage_factory {
+    fn make_sausage() {
+        println!("sausage!");
+    }
+}
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+fn main() {
+    sausage_factory::make_sausage();
+}
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+// Everything is private in Rust by default-- but there's a keyword we can use
+// to make something public! The compiler error should point to the thing that
+// needs to be public.