Delete mcb package. We have no need for it anymore.
Deletes mcbconv program. We have no need for it anymore.
All operations are performed directory on the original
minecraft map data.

The initial reasoning for the custom format and conversion
program, was a reduction in memory requirements, filesize
and more appropriate data layout. This turned out not to
work in practice. At least not for most common worlds.

Only flat worlds benefited from the conversion, due to their
grealy reduced number of blocks. But other worlds became
slow and unmanageable.
Fixes the Anvil package. The API is slightly simpler and it
gains some functionality needed by mcmapgen, which was
previously retained by mcb and mcbconv.
Converts mcmapgen to use anvil files directly through the
anvil package. No more reference to the mcb package should
exist. As it has been deleted.
Applies some further finetuning of the anvil API.
Fixes test cases.
Fixes mcmapgen in the aftermath of moving from mcb to anvil data.
Adds mcmapgen/world.go
This holds the World type, which was previously in the mcb package.
Fixes mcra package to reflect moving from mcb to anvil.