Add embedded-io, embedded-io-async, remove serial traits.

[Dirbaio]

As discussed previously and in today's meeting (chatlog, minutes), we've decided to not have serial traits, in favor of a more general "io" abstraction.

This PR moves the embedded-io crate to the embedded-hal repo. The crate currently living at https://github.com/embassy-rs/embedded-io, that repo will be archived once the move is complete.

Changes from the embassy repo:

• Removed defmt impls, needed for 1.0.
• Split embedded-io and embedded-io-async. Needed for 1.0, so we don't have to wait for AFIT to be stable.
• Removed async adapters for tokio and futures. I'm not sure if we want them here or it's better to leave them to community crates. If we do, I'll add them in a future PR.
• Added embedded-io::{ReadReady, WriteReady}. They're separate from the Read/Write traits since they can't be trivially implemented on top of std::io traits, so this would present a problem for the adapters. (It's likely implementable with e.g. some weird poll()/select() contraption, but that's OS-specific and I'm sure has other tradeoffs, so I'd rather not make the adapters do that by default.)

The crate has been added by parts in several commits to make review easier.

The embedded-io crate has been around for more than 1 year, so it's somewhat "proven". The design is the same as std::io, which is more than proven. Highlights of uses of embedded-io in the field (see more)

• embassy-nrf DMA-powered UART impl
• embassy-stm32 irq-driven UART
• embassy-stm32 DMA-powered UART
• embassy-rp irq-driven UART
• embassy-sync Pipe
• embassy-net TcpSocket
• embedded-tls TLS client, uses embedded-io to work on top of any TCP socket.
• reqwless HTTP client, uses embedded-io to work on top of any TCP socket or on top of embedded-tls.
• rust-mqtt, same.
• embedded-svc abstractions, widely used in the ESP32 ecosystem.
• embedded-nal-async models TCP sockets as Read+Write.
• es-wifi-driver, AT commands over uart
• esp8266-at-driver, AT commands over uart

[Dirbaio]

adressed @eldruin 's review comments