Part-specific meta-events

Fixed internal API

See #102
Part-specific meta-events

Fixed internal API for wrapping

TODO unwrapping and high-level

See #102
Prevents Chord.simultaneous from killing meta-events

See #102
Part-specific meta-events

Implemented, but does not work well with (</>) et al.

To fix this, we need the (HasPart Score) instance to apply to meta-events
as well.

See #102