DSL - make `length`/`duration` context sensitive

[jbarrett]

The word for being unavailable is a pain in the arse: Loop for 4 bars is a useful instruction. Keywords length and duration are sketchy and fuzzy right now.

loop foo => length '4 bars' => sub {
    every 'qn',
    note 'a4',
    length 'sn'
    ...
}

Have length add length to the event-element-in-progress, then always return it second parameter.