Add basic support for squashfs

[oxalica]

Fixes #830

The code mostly follows Zip implementations, using internal buffers for Seek requirement.

Unresolved questions:

1. Which extension to mention in docs? .squashfs or .sqfs?

   I'm currently picking sqfs.

   There is not canonical extension for squashfs, but there are two semi-official ones:

   • .squashfs: The most popular one from a quick search on GitHub. Also used by some distros.
     https://github.com/NixOS/nixpkgs/blob/6576d979e9a64d870b1f298a5c598116891a6c25/nixos/modules/installer/cd-dvd/iso-image.nix#L797
   • .sqfs: Mentioned in man mksquashfs by squashfs-tools, the reference implementation.
     https://github.com/plougher/squashfs-tools/blob/9f9bbd79016ff04967800f4301c7a9d0024c0c91/Documentation/manpages/mksquashfs.1#L494

2. backhand depends liblzma, a maintained xz2 fork, which conflicts with our dependency xz2. I disabled the Xz support of backhand to mitigate this. How can we fix this?

3. How to pass compression options for squashfs compressor? It is done inside squashfs format (per-chunk compression) thus does not use chained formats. We probably need a new CLI argument for it.

4. Currently, during compression, it buffers the whole squashfs in memory but we can write directly into file (which supports Seek). We did a special case in decompression, but it causes some code duplication and we does not do this for Zip compression either. Is there a better way to do so, or should we also add a special case in compression?

5. Do we want streaming compression support .sqfs.zstd (archive-then-compress instead or compress-per-chunk)? It could be done using some tricks like reserving spaces and emit ZSTD Literal Frame. Not sure about other compression formats. I don't think we want to support this, at least for now, given that decompression cannot be streamed anyway.

6. How should we support multiple input paths? Typically squashfs has a single "root directory". Currently, it reports an error if the input is not a single path to a directory. It would be weird to either "merge" all paths into one tree, or put multiple directory under the root directory (What mode/permission should the root use? Surprising behavior difference between one and more than one paths.).

[oxalica]

Seems the rest of clippy errors are on unrelated code.

The UI tests really tripped me. 😅 Seems a single cargo test ui - cargo insta review run will not fix all errors. It needs to be executed multiple times until there is no more. --no-default-features is also tested separately. We probably should document these better.

[marcospb19]

Hey, I apologize for the delay, I haven't been very active with Ouch, but I just wanted to let you know we'll be reviewing this properly, thanks for the patience!