Redesign vec1 to avoid confusing array literal

Learners were sometimes confused by the literal array. Their assumption
was that they are supposed to convert the array to a vector. Duplicating
the literal elements was not an intuitive solution.

This redesign avoids putting an identical array literal near the place
where learners are supposed to use the vec! macro.

solutions/05_vecs/vecs1.rs

@@ -1,10 +1,5 @@
-fn array_and_vec() -> ([i32; 4], Vec<i32>) {
-    let a = [10, 20, 30, 40]; // Array
-
-    // Used the `vec!` macro.
-    let v = vec![10, 20, 30, 40];
-
-    (a, v)
+fn elems_to_vec(a: i32, b: i32, c: i32) -> Vec<i32> {
+    vec![a, b, c]
 }
 
 fn main() {
@@ -16,8 +11,11 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_array_and_vec_similarity() {
-        let (a, v) = array_and_vec();
-        assert_eq!(a, *v);
+    fn test_elems_to_vec() {
+        let (a, b, c) = (2, 7, 12);
+        let v = elems_to_vec(a, b, c);
+        assert_eq!(v[0], a);
+        assert_eq!(v[1], b);
+        assert_eq!(v[2], c);
     }
 }