Expose Unix-compatible items in `std::os::unix`

[m-mueller678]

Currently, the standard library implements std::os::hermit::ffi::{OsStrExt,OsStringExt} traits, which have the same interface as the those exported under std::os::unix for other platforms. Re-exporting them under the same path would make many more crates compatible with hermit.

[mkroening]

This is because Hermit is not a Unix OS. It is becoming more and more similar, but historically, Hermit was not cfg(unix) in rustc.

We could think about making Hermit cfg(unix), but that would be a larger undertaking because we would have to gauge the ecosystem implications of all crates using Hermit as if it was a proper Unix system.

For the meantime, adjusting those crates for cases where you need OsStrExt is easier.

[mkroening]

We have tried this (hyperium/hyper-util#191 (comment)) but ended up deciding that cfg(unix) implies too much that we don't want to provide with Hermit currently, such as signals and spawning processes.

See rust-lang/rust#141838 (comment) for alternative future solutions to this.

[mkroening]

That said, changing the scope to exposing Hermit's Unix-compatible items in std::os::unix might be worthwhile.