Migrate the game to use an IBeatmap interface #2490
This is a half-proposal half-solution. Half-proposal because we may not need to go so far to use this interface game-wide - even where non-converted beatmaps are expected, but I think it makes sense to.
The idea behind this change is that we can substitute ruleset-specific beatmaps in-place of non-ruleset-specific beatmaps at any point in time. One particular use-case is beatmap statistics, for which I envision rulesets being able to return converted
E.g (upcoming PR):