Minor changes to NBT package plus debugging.

Turns out TAG_List's can have nil payloads -- who knew?  Minor cleanup
in readCompound() makes me think I'll have to do some more cosmetic
changes.  ReadTag() also now reports an error if no PReader is found.
Refactored world code to match requirements.  It works!

With a driver program, I was able to read an entire region file.  I
need to finish importing components of the driver program into the
readRegion function, but I wanted to check this in first.
Added initial support for entities and points.

In order to faithfully read and write worlds, I will need at least
some support for entities and such.  This is enough to get by for now.
There's also some support for points added, including tests.
Refactored WriteTag method.

It's actually a method, not a function, so I fixed it.
Cleaned up code and made it easier to understand.

Subject says it all.
Stash juggling, part one.

First attempts at reading worlds and region files involved stashing a
lot of code.  I need to commit what I have and then play with the stash.
Region files can now be read and written!

I was able to read a region file, make changes, and write it back, and
it worked.  Tests also pass.  They need to be beefed up severely, of
course.  The driver code needs to be migrated into the repository as
well.
First pass at world-reading driver.

Need to write a corresponding writer for worlds, then I can start
moving functionality from chunks and sections up to the world level as
appropriate.