add module3 exercise that shows uses of modules from std

exercises/modules/modules3.rs

@@ -0,0 +1,17 @@
+// modules3.rs
+// You can use the 'use' keyword to bring module paths from modules from anywere
+// and espically from the rust standard library. Bring SystemTime and UNIX_EPOCH
+// from the std::time module. Bonus style points if you can do it with one line!
+// Make me compile! Execute `rustlings hint modules3` for hints :)
+
+// I AM NOT DONE
+
+// TODO: Complete this use statement
+use ???
+
+fn main() {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
+        Err(_) => panic!("SystemTime before UNIX EPOCH!"),
+    }
+}

info.toml

@@ -368,6 +368,16 @@ its internal structure (the `fruits` and `veggies` modules and
 associated constants). It's almost there except for one keyword missing for
 each constant."""
 
+[[exercises]]
+name = "modules3"
+path = "exercises/modules/modules3.rs"
+mode = "compile"
+hint = """
+UNIX_EPOCH and SystemTime declared in the std::time module, add a use statement
+for these two to bring them into scope. You can use nested paths or the glob
+operator to bring these two in one line
+"""
+
 # COLLECTIONS
 
 [[exercises]]