Add slice exercise from @ruipserra!!! 💫✨

Connects to #29

primitive_types/primitive_types4.rs

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+// Get a slice out of Array a where the ??? is so that the `if` statement
+// returns true. Scroll down for hints!!
+
+fn main() {
+    let a = [1, 2, 3, 4, 5];
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+    let nice_slice = ???
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+    if nice_slice == [2, 3, 4] {
+        println!("Nice slice!");
+    } else {
+        println!("Not quite what I was expecting... I see: {:?}", nice_slice);
+    }
+}
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+// Take a look at the Primitive Types -> Slices section of the book:
+// http://doc.rust-lang.org/stable/book/primitive-types.html#slices
+// and use the starting and ending indices of the items in the Array
+// that you want to end up in the slice.
+
+// If you're curious why the right hand of the `==` comparison does not
+// have an ampersand for a reference since the left hand side is a
+// reference, take a look at the Deref coercions chapter:
+// http://doc.rust-lang.org/stable/book/deref-coercions.html