Enable NRTs across the whole project

[LumpBloom7]

This PR enables NRT, and adjusts the code to work with it, as it is quite a useful feature.

This is a draft as I'm still getting to grips with some of the conventions, along with waiting for the osu.Game.Rulesets namespace to fully use NRTs.

_{Not needing to specify canBeNull in BDL attributes is cool}