Fixes all the things :)

Hi!

I made a whole bunch of changes to the `nbt` library. It completely breaks the API as you know it, but it should be more performant (`Text` instead of `String`, `Array Int32` instead of `[]`) and usable (all `Maybe String` fields are gone, making it a bit easier to parse NBT-encoded structures).

I hope you find my changes useful. If you plan to merge, make sure to revert the changes I made to `README.md`.

Kind regards,

Stijn