Improve (Unmanaged)ArrayList.insert

(Unmanaged)ArrayList.insert has the same inefficiency as the old insertSlice. With the new addManyAt, the solution is trivial.
Also improves the test "growing memory preserves contents". In the previous implementation, if any changes were made to the ArrayList memory growth policy (function growMemory), the list could end up with enough capacity to not trigger a memory growth, defeating the purpose of the test. The new implementation more robustly triggers a memory growth.

Author: LucasSantos91

lib/std/array_list.zig

@@ -146,8 +146,8 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
         /// This operation is O(N).
         /// Invalidates pointers if additional memory is needed.
         pub fn insert(self: *Self, n: usize, item: T) Allocator.Error!void {
-            try self.ensureUnusedCapacity(1);
-            self.insertAssumeCapacity(n, item);
+            const dst = try self.addManyAt(n, 1);
+            dst[0] = item;
         }
 
         /// Insert `item` at index `n`. Moves `list[n .. list.len]` to higher indices to make room.
@@ -702,8 +702,8 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
         /// This operation is O(N).
         /// Invalidates pointers if additional memory is needed.
         pub fn insert(self: *Self, allocator: Allocator, n: usize, item: T) Allocator.Error!void {
-            try self.ensureUnusedCapacity(allocator, 1);
-            self.insertAssumeCapacity(n, item);
+            const dst = try self.addManyAt(allocator, n, 1);
+            dst[0] = item;
         }
 
         /// Insert `item` at index `n`. Moves `list[n .. list.len]` to higher indices to make room.
@@ -1761,33 +1761,33 @@ test "std.ArrayList/ArrayListUnmanaged.addManyAsArray" {
 }
 
 test "std.ArrayList/ArrayListUnmanaged growing memory preserves contents" {
+    // Shrink the list after every insertion to ensure that a memory growth
+    // will be triggered in the next operation.
     const a = std.testing.allocator;
     {
         var list = ArrayList(u8).init(a);
         defer list.deinit();
-        try list.ensureTotalCapacityPrecise(1);
 
         (try list.addManyAsArray(4)).* = "abcd".*;
-        try list.ensureTotalCapacityPrecise(4);
+        list.shrinkAndFree(4);
 
         try list.appendSlice("efgh");
         try testing.expectEqualSlices(u8, list.items, "abcdefgh");
-        try list.ensureTotalCapacityPrecise(8);
+        list.shrinkAndFree(8);
 
         try list.insertSlice(4, "ijkl");
         try testing.expectEqualSlices(u8, list.items, "abcdijklefgh");
     }
     {
         var list = ArrayListUnmanaged(u8){};
-        try list.ensureTotalCapacityPrecise(a, 1);
         defer list.deinit(a);
 
         (try list.addManyAsArray(a, 4)).* = "abcd".*;
-        try list.ensureTotalCapacityPrecise(a, 4);
+        list.shrinkAndFree(a, 4);
 
         try list.appendSlice(a, "efgh");
         try testing.expectEqualSlices(u8, list.items, "abcdefgh");
-        try list.ensureTotalCapacityPrecise(a, 8);
+        list.shrinkAndFree(a, 8);
 
         try list.insertSlice(a, 4, "ijkl");
         try testing.expectEqualSlices(u8, list.items, "abcdijklefgh");