Rollup merge of #161589 - pacak:private-osstr, r=clarfonthey

std: reduce visibility of some internal OsStr related types

The std::sys::os_str::{Buf, Slice} types are only used within the std crate and not actually exported. Whole `sys` module is private. They don't need to be public. This might result in a better generated code, but more importantly it avoids some compile errors down the line.

This commit is extracted from https://github.com/rust-lang/rust/pull/160971