Unify OsStringExt/OsStrExt traits across platforms

[thaliaarchi]

Currently, the std::os::{platform}::ffi::{OsStringExt, OsStrExt} traits have quite a bit of duplication, since there are several platforms using only three implementations of OsString/OsStr (bytes, WTF-8, and recently now UTF-8). To deal with this, the bytes platforms reexport the Unix ext traits, but provide their own module-level documentation. This documentation has grown slightly out of sync and some platforms forgot to include it entirely. UEFI, as it has much shared background with Windows, simply reexports the whole Windows ffi module.

Move these modules to std::sys::ffi::{bytes, windows, utf8} and publicly reexport them as each platform's std::os::{platform}::ffi module. To handle the differing documentation, use cfg_select!. The bytes impl has the examples in the module docs, to be reused, but could now be moved to the relevant traits. Since the Windows docs have much Windows-specific wording, I have named that module windows instead of wtf8, but if UEFI wants to grow its own documentation, we could rename it to wtf8 and create conditionally-included Markdown files for both, as I assume it would have many changes. For now, utf8 has Motor OS–specific docs, but I intend to generalize it when I add WASIp2 and Redox OS support (see #147932).

This is related to the effort in #117276, but takes a different approach by necessity of the traits and documentation being public, instead of an internal abstraction layer. I think it turned out reasonably and is better than what we currently have, but I am open to other approaches. The primary downside is that there are cfg gates that must be kept in sync in two places: in library/core/src/os/mod.rs and the docs of library/std/src/sys/ffi/bytes.rs.

The two-step diff makes it slightly more clear that the Unix traits were not changed; only moved.

r? @joboet

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