Binding events directly to an external bindable will cause that
bindable to hold a permanent reference to the current object.
We use `GetBoundCopy()` or otherwise a local bindable + `.BindTo()` to create a
weak-referenced copy of the target bindable. When the local bindable's
lifetime expires, so does the external bindable's reference to it.
In preparation to remove `DistancedHitObjectComposer`, split off
`IPositionSnapProvider` from `IDistanceSnapProvider`.
`DistancedHitObjectComposer` was not touching `IPositionSnapProvider`'s
only interface member at all, it was just forwarding it for subclasses
to override to their own leisure.