Refactor this abstractions to allow for native (via ffi) or pure dart implementations.

[sethloco]

es_compression was always designed with the intent of using ffi for algorithm implementations.
However, it may be good to keep the door open for algorithm implementations in pure dart so it can be used in other contexts.

This means a slight refactor to CodecBuffer

• Its _bytes is a Pointer. Not a big change to parameterize this

The current lz4 decoder/encoder/options refer to the lz4 c library constants and dispatcher.