Rust portable-* crate placeholders

[brodycj]

Existing portable-atomic & portable-atomic-util crates are really nice packages to get atomic operations & Arc working on embedded platforms that may not have atomic operations built-in. I have reserved some more crate names to get some more Rust std features working on similar embedded platforms:

• portable-arc - may be used to support Arc with nightly Rust features
• portable-atomic-arc - more explicit name
• portable-io - std::io features, see this for a working concept working for rustls: https://github.com/rustls/rustls/pull/2137
• portable-io-error - may be used for std::io::Error support
• portable-alloc
• portable-std