factor out codecs and core traits

[koehlma]

This PR fixes #324.

It introduces two new crates:

• compression-core: Contains the Decode and Encode traits.
• compression-codecs: Contains the former codec module.

I ran the changes through cargo semver-checks and it did not complain.

Along the way, I made some decisions that may require further discussion:

• The root crate async-compression does not expose any of the data structures of the other crates. In particular, I did not move the Level enum and the modules zstd and brotli into compression-codecs. This enables future breaking changes in compression-codecs without breaking async-compression.
• The crate compression-codecs re-exports the underlying compression libraries. The root crate does not directly depend on any of the libraries anymore. I migrated everything, including the tests, to the re-exported libraries. It may be worthwhile to add additional abstractions to compression-codecs to hide the underlying libraries. However, I think, that this should not be part of this PR, as it is already rather huge.

[NobodyXu]

Thank you!

Some feedback on the code