Fix crash on \rest when the note completes the bar

[FelippeRoza]

The example in #147 crashes with IndexError: list index out of range in Bar.inject_voice(). Fixes #147.

note2rest() popped the note off self.bar, but \rest is parsed after the note it applies to, so when that note completed the bar self.bar had already moved to the next one. Three things went wrong at once: the note stayed a sounding note, the next bar lost its BarAttr to the pop(), and it gained a stray rest.

add_to_bar() now records which bar it put an object in, and note2rest() converts the note there. The rest keeps its <display-step>, which is the point of \rest.

Note this is the same shape of bug as the chord half of #193, which remembers the bar in new_chordbase() for the same reason. If both land, chord_bar there could use the obj_bar added here.