Extend `EncodedChunk` abstraction to write side

[kylebarron]

Have

Array.encode_chunk() -> EncodedChunk

which takes in chunk indices and a typed, decoded array (#47).

Or rather, because chunk encoding will be either sync (current thread) or async (on a Rust thread pool), maybe have a separate ChunkEncoder class.

Or, just put encoding onto CodecChain, so we have CodecChain.encode and CodecChain.encode_async.

• We probably want a way to encode/decode chunks that is independent of a store.
• It seems a little messy to put sync/async methods for chunk encoding onto Array/ AsyncArray, when the sync/async bridge of chunk encoding describes where the CPU bound work is happening, not what kind of IO is happening