Implement lz4 stream mode

[jheyens]

As of v1.23, the lz4 frame and block operations are implemented. Implementing bindings for the third mode of operation, lz4 streaming is the next step for this crate.

I probably won't start implementing it in the next few days, so I haven't given much thought on how the API should look like. Feel free to propose something. Currently, I suppose the encoder/decoder pattern of the lz4 frame mode is a good idea, since the streaming api is meant to be used with rather large compressed sizes and network streams.