Please support enabling zstd long-distance matching mode about async-compression

[joshtriplett]

I submitted #134 implementing this. (Happy to take a different approach if you would prefer.)

[Nemo157]

Unfortunately I want to keep the algorithmic dependencies of async-compression private to allow non-breaking updates.

I have long been meaning to split out another crate with the underlying generic implementation that exposes these and would allow full configurability for different libraries, with the downside of needing to either support multiple implementations/versions of the algorithms or doing breaking releases.

I think I had started an attempt at that a while ago, but found it less trivial than I was hoping. This is some motivation to look at it again, hopefully I can at least see how difficult it appears this weekend.

[joshtriplett]

Would you consider a PR adding an abstraction over a handful of the options, to avoid exposing the underlying library? I'd be happy to prepare such a patch.

[joshtriplett]

This is now fixed, with #134.