Redo dt calculation, general Kernel refactor

[girvel]

Just totally redo how the time is counted in our love.run implementation

• Bug: saving time goes to systems' dt
• Bug: time of the window being inactive goes to systems' dt
• love.timer.getTime() also doesn't consider that; should probably do State.getTime()
• Also, in-game time should be counted in the State so it can be saved/loaded; the app-time should stay in the kernel level (if it's even needed)

[girvel]

Actually, there should be 3 different time counters:

1. Kernel time: counts the time the program is opened (actually, it's done with os.clock, but we can measure it just for the log report)
2. State time: counts the time from the start of the game, including the time when the game is loading (for more technical calculations), serialized?
3. State game time: counts the time in-game, excluding loading/saving time

Probably wouldn't need the dt argument anymore

Also, do we even need a State with .is_loaded = false? Maybe we should hardcode non-live systems into kernel, and there's no awkward two-stage initialization for State, and we would be able to serialize asynchronously, because State = nil during all saving/loading (though, the picture would still be frozen, ECS would be inactive).

[girvel]

Creating a Kernel is not the same as assigning wrappers/callbacks, and are done separately, one is a factory, the other is module-script