ENH: Introduce np.arr array creation in `numpy.mat` style. Fixes #4817.

Introduces `np.arr(s)` which uses a utility function from ``numpy.mat`` to implement array creation in `numpy.mat` style. In #4817 @frozen-flame suggested this in light that it would be beneficial to beginners and presenters during demonstrations.

Examples:

    >>> np.arr('3; 4; 5')
        array([[3],
               [4],
               [5]])
    
    >>> np.arr('3; 4; 5', dtype=float)
        array([[ 3.],
               [ 4.],
               [ 5.]])
    
    >>> np.arr('1 0 0; 0 1 0; 0 0 1')
        array([[1, 0, 0],
               [0, 1, 0],
               [0, 0, 1]])

    >>> np.arr('4, 5; 6, 7')
        array([[4, 5],
               [6, 7]])