Switch layered to overlay filesystem

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

This PR removes the old layered file system, which had to deal with two different semantics with non-trivial complexity, instead to an overlay-based file system. This is aided by work I'd done to set up the ability to mount things (via the composer, #995), as well as a backend trait more amenable to the necessary composition (Backend, #887). With this PR, the last of the remaining layered file systems are removed.

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

As with many of my prior PRs, this PR is easiest to review one commit at a time :)

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/enum_missing.ron

Failed in:
  enum litebox::fs::layered::LayeringSemantics, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/4a13b150b988bc40f65f7914a9d2aabd019f4549/litebox/src/fs/layered.rs:29
  enum litebox::fs::layered::MigrationError, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/4a13b150b988bc40f65f7914a9d2aabd019f4549/litebox/src/fs/layered.rs:425

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/module_missing.ron

Failed in:
  mod litebox::fs::layered, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/4a13b150b988bc40f65f7914a9d2aabd019f4549/litebox/src/fs/layered.rs:4

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/struct_missing.ron

Failed in:
  struct litebox::fs::layered::FileSystem, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/4a13b150b988bc40f65f7914a9d2aabd019f4549/litebox/src/fs/layered.rs:56