Move `OsStringExt` and `OsStrExt` to `std::os`
Moves the `OsStringExt` and `OsStrExt` traits and implementations from `sys_common` to `os`. `sys_common` is for abstractions over `sys` and shouldn't really contain publicly exported items.
This does introduce some duplication: the traits and implementations are now duplicated in `unix`, `wasi`, `hermit`, and `sgx`. However, I would argue that this duplication is no different to how something like `MetadataExt` is duplicated in `linux`, `vxworkx`, `redox`, `solaris` etc. The duplication also matches the fact that the traits on different platforms are technically distinct types: any platform is free to add it's own extra methods to the extension trait.