Perf optimizations

With the --fast flag gifski is now about as quick as ffmpeg in wall-clock time*, but still generates files that are smaller and look better at the same time. *(tested on M1 CPU, so YMMV)

The latest version performs LZW encoding of frames in parallel, and avoids using most of gifsicle codebase except the lossy LZW compression part. PNG decompression and resizing now also run on a threadpool even when used via gifski's C API.

gifski can now make a good use of 8 CPU cores, which a serious overengineering of the sequential GIFs encoding, but I'm using Rust, so I just had to throw multi-threading at every problem.