This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
LegacyDifficultyCalculatorBeatmapDecoder was registered as a fallback
decoder in commit ffde389 for future use in the server-side difficulty
calculation components. Due to the pre-existing fallback registrations
this causes a runtime crash when the diffcalc components are started.
Add a test reproducing this scenario to prevent the issue from
resurfacing in the future.