Encode block changes using simple RLE

[caszuu]

See #44.

This impl differs from the proposal in two ways:

• it implements the coord-index offseting as an optional flag so both continually moving or unmoving runs can be compressed
• it compresses changes over the entire BlockChange structs instead over its individual members due to complexities in spliting multiple overlapping RLE runs at once.

Only for benchmarking so far, both chest support and serialization support is missing but could be semi-easily added.