Implement random mod for osu!standard

[MaxOhn]

Commit https://github.com/MaxOhn/rosu-pp/pull/51/commits/6d4058f240e1563ce5bd537ebda1620a547a2a46 introduced random mod implementations for taiko and mania but osu!standard was left out because its implementation relies on hitobjects' combo index which is never stored as of now.

If difficulty or performance calculation in the future ever relies on combo indices, making it mandatory to parse them out of .osu files, the random mod can be implemented for osu!standard as well. Otherwise, I don't think it's worth parsing out combo data just for the random mod which presumably occurs only very rarely.