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.
Previously the slider path length would be snapped using the current
beat snap setting on *every* change of the slider path. As it turns out
this is unexpected behaviour in some situations (e.g. when reversing a
path, which is expected to preserve the previous duration, even though
the slider may be technically "unsnapped" at that point in time due to a
different beat snap setting being selected afterwards).