Rollup merge of #138832 - ChrisDenton:with_native_path, r=joboet

Start using `with_native_path` in `std::sys::fs`

Ideally, each platform should use their own native path type internally. This will, for example, allow passing a `CStr` directly to `std::fs::File::open` and therefore avoid the need for allocating a new null-terminated C string.

However, doing that for every function and platform all at once makes for a large PR that is way too prone to breaking. So this PR does some minimal refactoring which should help progress towards that goal. The changes are Unix-only and even then I avoided functions that require more changes so that this PR is just moving things around.

r? joboet

Author: matthiaskrgr