Add substring API for `OsStr` This adds a method for taking a substring of an `OsStr`, which in combination with [`OsStr::as_encoded_bytes()`](https://doc.rust-lang.org/std/ffi/struct.OsStr.html#method.as_encoded_bytes) makes it possible to implement most string operations in safe code. API: ```rust impl OsStr { pub fn slice_encoded_bytes<R: ops::RangeBounds<usize>>(&self, range: R) -> &Self; } ``` Motivation, examples and research at https://github.com/rust-lang/libs-team/issues/306. Tracking issue: #118485 cc `@epage` r? libs-api