Fix various world loading problems (Issue #265)

[Schuwi]

This PR aims to fix a few issues I encoutered when trying to load a vanilla world. Mainly this fixes the format of heightmaps in chunk data.
This is basically ready to merge, just needs a little polish. Namely:

• Remove panics from packed_u9.
  (I just can't figure out how to return an error from serialize or deserialize. I thought serde::ser::Error::custom("my error") would do the trick but rustc says nope.)
• Complete the test implementation for packed_u9/Heightmaps::packed_i64_vec.
• Run cargo fmt and check cargo clippy.

Resolves #265.

[Schuwi]

I spent waaaay to much time on those packed u9 heightmap arrays. Finally everything works, tests are implemented and deserializing a vanilla chunk gives sensible heightmap data.
This should now be ready for merge and allow for some sweet vanilla world loading once again.

[Schuwi]

Huh, I just can't replicate that clippy warning. Even running explicitely with cargo clippy -- -D clippy::inconsistent-digit-grouping executes without warnings. I'll just add a #[allow(clippy::inconsistent-digit-grouping)] because the digit grouping makes sense here.

[caelunshun]

Thanks, this is great work!

[Schuwi]

Thanks for all the comments! I'm sorry there had to be so many. I got a bit sloppy in the end and just wanted to be done with it. This turned into a way bigger thing than I thought it would.
But now that PistonDevelopers/hematite_nbt#52 is merged I can fully concentrate on this and I'll take a look at your comments tomorrow with a clear head :)

[Schuwi]

Alright, so I have actually implemented serde serialization for chunk data in my local working copy now and everything works. I am not sure whether to include it in this PR or put it into a separate one. That would also fix the server panicking when failing to save a chunk and put out a warning instead.

[caelunshun]

Alright, so I have actually implemented serde serialization for chunk data in my local working copy now and everything works. I am not sure whether to include it in this PR or put it into a separate one. That would also fix the server panicking when failing to save a chunk and put out a warning instead.

Nice, thanks for that! I think the new serialization probably belongs in another PR—this PR is pretty much ready to merge.

[Schuwi]

Alright, then everything else would be for another PR so this would be ready to merge from my side.

[caelunshun]

Great, thank you!