Consider better implementation for chains

[rigdern]

I went with the most straightforward implementation. It just omits any chains that don't fully fit into the song slice.

Improvement ideas:

• If a chain doesn't fully fit into a slice, it could be truncated to the portion that does fit into the slice. Would this benefit be worth the implementation cost?

• Whenever we omit a SliderData that represents a chain (i.e. its sliderType is Burst) then we should also omit the item representing the head of the chain (the head of the chain is a NoteData with type: BurstSliderHead).

  For now, we leave the head of the chain alone because it results in the simplest implementation. Leaving it in doesn't result in any undesirable side effects in the game, as far as I know.

  A good question is: is there any noticeable impact of our current implementation of omitting the SliderData but not NoteData, type: BurstSliderHead? If not, perhaps it’s not worth the implementation cost of doing anything fancier than we’re currently doing.