Use zig zag encoding for the chunk x/z ints

[mworzala]

VarInts are not great for negative twos-complement ints. It would generally be an improvement to zig zag encode those, though it will be a minor difference.

For example, saving 4 chunks around 0-0, there would be 64 coordinates saved. Using ints this would be 32*4 = 128 bytes. Using VarInts the positive ones would be 1 byte, negative ones would be 5 bytes, so 16*1 + 16*5 = 81 bytes. Using zig zag encoding + var ints it would be 1 byte for each value, so 32 bytes.