factor out codec into a separate crate

[koehlma]

I would like to use the abstractions provided for the various compression algorithms in the codec module in a synchronous context. Would you be open to factoring out the codec module into a separate crate? I would be glad to work on a corresponding PR.

[NobodyXu]

I personally think that's a reasonable and willing to accept such PR, cc @robjtede @Nemo157 what's your thoughts on this?

[Nemo157]

That's something I'd thought of doing for a while but never got round to. I was also thinking it'd be good to extract the generic state machine IO impls into a separate crate too, that way it would be possible to implement codec for an unsupported compression algorithm and wrap it in a reader/writer.

[koehlma]

I opened a PR also factoring out the core IO traits as suggested by @Nemo157.

For some reason, the Gzip test with level 0 is failing. However, this also seems to happen with the latest upstream version (on my machine). Any ideas?

[NobodyXu]

For some reason, the Gzip test with level 0 is failing. However, this also seems to happen with the latest upstream version (on my machine). Any ideas?

Just slim through the PR, not sure about that