Incorporate Move Semantics exercises from Felix's tutorial

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Connects to #6

move_semantics/move_semantics3.rs

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+// Make me compile without adding new lines-- just changing existing lines!
+// (no lines with multiple semicolons necessary!)
+// Scroll down 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> {
+    vec.push(22);
+    vec.push(44);
+    vec.push(66);
+
+    vec
+}
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+// The difference between this one and the previous ones is that the first line
+// of `fn fill_vec` that had `let mut vec = vec;` is no longer there. You can,
+// instead of adding that line back, add `mut` in one place that will change
+// an existing binding to be a mutable binding instead of an immutable one :)