Add some exercises about Strings and &strs!

strings/strings3.rs

@@ -0,0 +1,18 @@
+// Ok, here are a bunch of values-- some are `Strings`, some are `&strs`. Your
+// task is to call one of these two functions on each value depending on what
+// you think each value is. That is, add either `string_slice` or `string`
+// before the parentheses on each line. If you're right, it will compile!
+
+fn string_slice(arg: &str) { println!("{}", arg); }
+fn string(arg: String) { println!("{}", arg); }
+
+fn main() {
+    ("blue");
+    ("red".to_string());
+    (String::from("hi"));
+    (format!("Interpolation {}", "Station"));
+    (&String::from("abc")[0..1]);
+    ("  hello there ".trim());
+    ("Happy Monday!".to_string().replace("Mon", "Tues"));
+    ("mY sHiFt KeY iS sTiCkY".to_lowercase());
+}