Switch to explicit file system Contexts

[Jay Bosamiya (Microsoft) (jaybosamiya-ms)]

This PR switches the file system resolver to explicit Contexts, so that the underlying file system(s) and the context that they are used in are separated. Essentially, this means that nothing within the file system is itself aware of CWD (current working dir) or acting user now, and the Context object explicitly carries this. This means that the Linux shim no longer needs to maintain its own cwd: String field and manipulation of it, allowing resolution + permission decisions to live in one place.

Along with this, I also updated the in-mem backend to use the resolver context rather than maintain its own user management, closing out yet another place of unnecessary duplication and potential inconsistency.

Finally, as a drive-by fix: getcwd no longer returns a trailing /, making it more consistent with Linux.

[Jay Bosamiya (Microsoft) (jaybosamiya-ms)]

Again, easiest to review one commit at a time :)

[github-actions]

ℹ️ Note: This semver check was run against the jayb/retire-filesystem-trait branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox::fs::resolver::Resolver::open takes 3 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:438, but now takes 4 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:469
  litebox::fs::resolver::Resolver::chmod takes 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:787, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:833
  litebox::fs::resolver::Resolver::chown takes 3 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:800, but now takes 4 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:848
  litebox::fs::resolver::Resolver::unlink takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:818, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:869
  litebox::fs::resolver::Resolver::mkdir takes 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:843, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:893
  litebox::fs::resolver::Resolver::rmdir takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:868, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:926
  litebox::fs::resolver::Resolver::file_status takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_retire_filesystem_trait/9974d69755a0316e6af624bb51f0b21013463175/litebox/src/fs/resolver.rs:932, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox/src/fs/resolver.rs:989