Single threaded option - rayon as the default feature

[apps4uco]

I know the crate is specifically for multi-threaded encoding/decoding.
I have managed to get sub millisecond encoding per image for my use case of encoding hundreds of small png files concurrently, and I would like to use mtpng to have low level control over Indexed pngs with transparency.

However, server side I do not want to use many threads on each request. Throughput of the server is more important, not time per request, so I think using the current thread would be the best way to do this.

I have looked at the code to see how easy it would be to have rayon as a default (optional) dependency, and be able to add default-features=false. However, I dont understand the code enough to remove the multithreading part in encoder.rs.

Also Im not even sure there would be a significant performance gain over

let pool = rayon::ThreadPoolBuilder::new().num_threads(1).build().unwrap();
(except that creating a thread pool per request seems like a bad idea)

I'd like to get feedback on this, also it could be useful for the WASM issue #13

[bvibber]

Hmmmmmm, well honestly it should be possible to gate stuff on a platform/feature, and it may indeed be useful to have a common API between threaded and non-threaded versions of apps. I'll keep this in mind for my upcoming refactor. :)

I'm planning to move the deflate compression portion out to a separate library, so that's a great chance to do some cleanup on the guts while I'm moving it around. :D