What & why

The processkit::Error enum takes over 100 bytes, bloating every Result and triggering clippy lints. See issue #21 for details.

This is the suggested fix, moving all enum variants to a new processkit::ErrorKind struct and making processkit::Error an opaque struct wrapping Box<ErrorKind>.

Checklist

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings is clean
• cargo test passes (and cargo test --all-features -- --ignored if you
  touched spawn / containment / streaming paths)
• CHANGELOG.md [Unreleased] updated when the change is user-facing
  (Added / Changed / Fixed)
• Docs updated (rustdoc and the docs/ guide set) if behavior or API changed
• (N/A) New dependencies carry a "why" comment in Cargo.toml (see CONTRIBUTING.md)

Notes for reviewers

There is some code that could be cleaner if we used if let guards in match.
This would require bumping the MSRV to Rust 1.95.

Right now in src/runner.rs around line 810 we do

let mut child = match group.spawn_with_options(&mut tokio_cmd, &opts) {
    Ok(child) => child,
    Err(e)
        if matches!(e.kind(), crate::ErrorKind::Spawn { source, .. } if source.kind() == std::io::ErrorKind::NotFound) =>
    {
         // some code
    },
    Err(other) => return Err(other)
};

If we bump the MSRV to 1.95, this could change into the following code:

let mut child = match group.spawn_with_options(&mut tokio_cmd, &opts) {
    Ok(child) => child,
    Err(e)
        if let crate::ErrorKind::Spawn { source, .. } = e.kind()
            && source.kind() == std::io::ErrorKind::NotFound => {
        // some code
    },
    Err(other) => return Err(other)
};