Measure repr is now evaluable.

Old:

    >>> Measure((3, 8), "c'8 d'8 e'8")
    Measure(3/8, [c'8, d'8, e'8])

New:

    >>> Measure((3, 8), "c'8 d'8 e'8")
    Measure((3, 8), "c'8 d'8 e'8")