Official ESP-IDF framework support

Initial request in #1691, initial addition in https://github.com/tokio-rs/mio/pull/1692.

Requirements:

• Committed maintainer(s).
• Some sort of CI support, preferably cargo test, cargo check will do.

To do after:

• Document specifics, e.g. in Poll and Event.
• Document supported versions (we don't do this properly yet, might as well start at some point).