Support ESP-IDF framework (esp32 family of hardware devices with std support)

[jasta]

Is your feature request related to a problem? Please describe.
ESP32 family of devices (e.g. ESP32C3) can work with tokio but requires patches to socket2, mio, and tokio itself.

Describe the solution you'd like
Generally, fix build errors in socket2 and tokio due to missing functionality, then merge in poll support to mio. This list tracks the dependencies:

• mio: Initial support for poll-based backend mio#1687
• mio: Add ESP-IDF framework support mio#1692
• socket2: Support for the ESP-IDF framework rust-lang/socket2#452
• socket2: Small fix for ESP-IDF platform support rust-lang/socket2#460
• tokio: Add cooperative UCred impl for espidf and other process-less OSes #5868
• tokio: poll: Do not clear readiness on short read/writes. #5881

Describe alternatives you've considered
The esp community currently is favoring smol/async-io but still has to maintain non-upstream patches. An alternative would be to continue on this path, however I prefer adding tokio support directly for two reasons:

1. It allows libraries built against tokio features="net" to still interop (right now they theoretically can but since they don't compile it's a non-starter).
2. tokio has more community support with great tools like tokio console.

[jasta]

All PRs are submitted now. Just waiting on reviews...

[jasta]

Woohoo! Fully merged, latest tokio master branch should support the ESP32 family but with a quirk that you'll need to modify .cargo/config.toml:

[target.riscv32imc-esp-espidf]
rustflags = ["--cfg", "mio_unsupported_force_poll_poll"]

And to patch Cargo.toml you'll need:

[patch.crates-io]
mio = { git = "https://github.com/tokio-rs/mio", branch = "master" }
socket2 = { git = "https://github.com/rust-lang/socket2", branch = "master" }
tokio = { git = "https://github.com/tokio-rs/tokio", branch = "master" }

EDIT: I'm working on a bare bones github repo that shows this all working

[jasta]

Added a demo repo to show it all in action: https://github.com/jasta/esp32-tokio-demo