Merge decompressor into this repo

[nyurik]

Is there any value in keeping two tightly-coupled repositories separate: the rust-brotli and rust-brotli-decompressor? It seems like they should just be two separate crates as sub-dirs in the same repository, using workplace cargo feature to manage all connections, and it can still be published separately if needed. This way all changes would go in in sync, and a single git's revision would indicate the state of all brotli code.