[stdlib] Add array_tester with Tester and Testee for lists

[rd4com]

Introducing two new types for testing lists and datastructures.
(Tester and Testee)

Testee can be used as the element type of an array or a similar datastructure.
Tester holds Int fields to count the amount of moves, copy, init, del.

Multiple instances of Testee contains an UnsafePointer to an instance of Tester.
Assertions can be created on the current amount of init, copy, del, moves a list does.

Additionally, Testee have an Int field to store a value.
That value can be used to check the equality of the elements of two lists.

Example:

def main():
    var _tester = Tester()
    var x = List[Testee]()
    for i in range(10):
        x.append(Testee(_tester, i))
    for i in x: print(i[].value)
    assert_equal(_tester.oninit, 10)
    assert_equal(_tester.ondelete, 0)
    assert_equal(_tester.oncopy, 0)
    y = x[0]
    assert_equal(y.value, 0)
    assert_equal(_tester.oncopy, 1)
    _ = x.pop()
    assert_equal(_tester.ondelete, 2)
    __type_of(x).__del__(x^)
    assert_equal(_tester.ondelete, 11)
    assert_equal(_tester.oncopy, 1)

[JoeLoser]

Thanks for the contribution! Can this be actively applied to any existing stdlib tests, or what's the motivation compared to some of the other test utils we have?