Varint NBT support

[ghost]

Don't merge, not ready yet.

[rom1504]

the reformatting make the diff longer than necessary

[rom1504]

ok so :

1. does it work ?
2. this needs some adapting to have both normal nbt, and new-mcpe nbt (and old-mcpe nbt) : I can do that

[ghost]

strings, it does. I thought I made everything else work but it still doesn't decode the bigtest. I'll work on this when I get a chance, because it's main purpose is for extra data and NBT data on chunks