cap-walkdir

[`walkdir`] is a popular cross platform Rust library for efficiently walking a directory recursively. It'd be interesting to either wrap or port this crate to `cap-std` to allow recursively walking the subtree of a `Dir`.

[`walkdir`]: https://crates.io/crates/walkdir

Start by reviewing cap-std's existing Dir APIs and comparing them with the walkdir crate's recursive traversal behavior. Decide whether wrapping or porting is appropriate for capability-scoped subtree walking, then define completion around a usable recursive walker for Dir with coverage of the relevant traversal behavior.

Written by the indexing model from the issue text.