This implements assertions/warnings supported by the module system directly,
instead of just being a NixOS option (see
nixos/modules/misc/assertions.nix).
This has the following benefits:
- It allows cleanly redoing the user interface. The new
implementation specifically allows disabling assertions or
converting them to warnings instead.
- Assertions/warnings can now be thrown easily from within
submodules, which previously wasn't possible and needed workarounds.
df5ba82f74