Add block/meta and item/meta pair classes

[RecursivePineapple]

This PR adds a few util classes that I've needed several times in different mods. I've also seen similar classes replicated a good amount of times, so I figure these should be formalized properly.

If the mutability contract isn't clear, let me know. I tried to make it as robust as possible.

[YannickMG]

Could you offer an example use case where we'd want to have a BlockMeta instance which can be mutated outside the control of the code that receives it? Worse, there's nothing stopping anyone from returning a mutable BlockMeta when a ImmutableBlockMeta is required so you can actually never guarantee the immutability of the data you receive. That sounds like a recipe for bugs.

It seems to me we'd be much better of and with a cleaner API with only immutable implementations.

If you must have both, they probably shouldn't share an interface.

[RecursivePineapple]

Ideally there'd be a way to statically check the mutability, but since we're writing java and not rust I had to rely on this system. I can add an immutable implementation if that'd be better, but I didn't think it was necessary. The only benefit that would provide is to prevent casting down to BlockMeta, which people shouldn't do unless they have a very good reason (in which case, they should be able to mutate it).

Mutability is useful for a pattern similar to pointers. It's mainly a way of reducing allocations in hot-paths. 99% of the time you'll want to return or accept an immutable pair unless you have a good reason not to, since that's the more generic type.

ImmutableBlockMeta getSurfaceBlock(World world, int x, int z) {
  BlockMeta pooled = new BlockMeta(Blocks.air, 0);

  for (int y = 255; y >= 0; y--) {
    getBlock(pooled, world, x, y, z);

    if (pooled.getBlock().getMaterial() = Materials.rock || pooled.getBlock().getMaterial() == Materials.ground) {
      pooled.setBlockMeta(world.getBlockMetadata(x, y, z));
      return pooled;
    }
  }
 
  return null;
}

void getBlock(BlockMeta bm, World world, int x, int y, int z) {
  // as an example, this would need to be much more complex
  bm.setBlock(world.getBlock(x, y, z));
}

[github-actions]

Warning: 2 uncommitted changes
#99