Incorporate Move Semantics exercises from Felix's tutorial

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Connects to #6

move_semantics/move_semantics4.rs

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+// Refactor this code so that instead of having `vec0` and creating the vector
+// in `fn main`, we instead create it within `fn fill_vec` and transfer the
+// freshly created vector from fill_vec to its caller. Scroll for hints!
+
+pub fn main() {
+    let vec0 = Vec::new();
+
+    let mut vec1 = fill_vec(vec0);
+
+    println!("{} has length {} content `{:?}`", "vec1", vec1.len(), vec1);
+
+    vec1.push(88);
+
+    println!("{} has length {} content `{:?}`", "vec1", vec1.len(), vec1);
+
+}
+
+fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
+    let mut vec = vec;
+
+    vec.push(22);
+    vec.push(44);
+    vec.push(66);
+
+    vec
+}
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+// Stop reading whenever you feel like you have enough direction :) Or try
+// doing one step and then fixing the compiler errors that result!
+// So the end goal is to:
+// - get rid of the first line in main that creates the new vector
+// - so then `vec0` doesn't exist, so we can't pass it to `fill_vec`
+// - we don't want to pass anything to `fill_vec`, so its signature should
+//   reflect that it does not take any arguments
+// - since we're not creating a new vec in `main` anymore, we need to create
+//   a new vec in `fill_vec`, similarly to the way we did in `main`