[WIP] ESP32 support

[simlay]

The ESP32 supports using the stdlib so I figured I'd give this a shot. It's still unclear if this will work at all or if it will bloat the embedded binary too much.

At the time our authorship (I might not keep these up to date), here's some of the pieces I had to remove:

• get-if-addrs needs a replacement
• libmdns needs to be swapped out to use esp-idf-svc's mDNS service
• The mio crate doesn't build for this target and so tokio::net::TcpStream isn't available so the AsyncReader/AsyncWriter logic needs reworking.