--- compact
--- compact!

    compact  self  nil ǤǤ
    ֤ޤcompact! ѹ˲Ū˹Ԥѹ
    Ԥ줿 self 򡢤Ǥʤ nil ֤ޤ

    :
        ary = [1, nil, 2, nil, 3, nil]
        p ary.compact   #=> [1, 2, 3]
        p ary           #=> [1, nil, 2, nil, 3, nil]
        ary.compact!
        p ary           #=> [1, 2, 3]
        p ary.compact!  #=> nil

