Add some exercises about Strings and &strs!

strings/strings1.rs

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+// Make me compile without changing the function signature! Scroll down for hints :)
+
+fn main() {
+    let answer = current_favorite_color();
+    println!("My current favorite color is {}", answer);
+}
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+fn current_favorite_color() -> String {
+    "blue"
+}
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+// The `current_favorite_color` function is currently returning a string slice with the `'static`
+// lifetime. We know this because the data of the string lives in our code itself -- it doesn't
+// come from a file or user input or another program -- so it will live as long as our program
+// lives. But it is still a string slice. There's one way to create a `String` by converting a
+// string slice covered in the Strings chapter of the book, and another way that uses the `From`
+// trait.

strings/strings2.rs

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+// Make me compile without changing the function signature! Scroll down for hints :)
+
+fn main() {
+    let guess1 = "blue".to_string(); // Try not changing this line :)
+    let correct = guess_favorite_color(guess1);
+    if correct {
+        println!("You guessed correctly!");
+    } else {
+        println!("Nope, that's not it.");
+    }
+}
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+fn guess_favorite_color(attempt: &str) -> bool {
+    attempt == "green"
+}
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+// Yes, it would be really easy to fix this by just changing the value bound to `guess1` to be a
+// string slice instead of a `String`, wouldn't it?? There is a way to add one character to line
+// 5, though, that will coerce the `String` into a string slice.

strings/strings3.rs

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+// 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());
+}