Add set index in list, and update lib+examples+tests

tests/list.ang

@@ -34,3 +34,10 @@ assertEq("the, first, time".split(", "), ["the", "first", "time"])
 # list indexing
 assertEq([1, 2, 3].at(1), 2)
 assertEq(["a", "b", "c"].at(2), "c")
+
+# mutating list
+a := [1, 2, 3]
+assertEq(a, [1, 2, 3])
+
+a[1] = 4
+assertEq(a, [1, 4, 3])

tests/tuple.ang

@@ -9,3 +9,10 @@ fn neighbours(n: int) -> (int, int) {
 }
 
 assertEq(neighbours(2), (1, 3))
+
+a := (1, 2)
+assertEq(a, (1, 2))
+
+(x, y) := a
+assertEq(x, 1)
+assertEq(y, 2)