Changed interpreter representation of contexts again.

OLD:

    >>> voice_1 = Voice("c'8 d'8 e'8")
    >>> voice_2 = Voice("c'4.")
    >>> Staff([voice_1, voice_2])
    Staff{2}

NEW:

    >>> Staff([voice_1, voice_2])
    <Staff{2}>

The angled brackets show that the interpreter representation is nonevaluable.