Allow limited access to `OsStr` bytes `OsStr` has historically kept its implementation details private out of concern for locking us into a specific encoding on Windows. This is an alternative to rust-lang#95290 which proposed specifying the encoding on Windows. Instead, this only specifies that for cross-platform code, `OsStr`'s encoding is a superset of UTF-8 and defines rules for safely interacting with it At minimum, this can greatly simplify the `os_str_bytes` crate and every arg parser that interacts with `OsStr` directly (which is most of those that support invalid UTF-8). Tracking issue: #111544