Tokio v1

[mrdxxm]

Issue #13

This is pretty big changes to async part of API and I'm noob in tokio infrastructure, so it's definitely needs a picky review and further testing.

I tested it only in devcontainer on aarch64 so long.

VPN example turned out to be tricky to adapt, so I'll do it later.

MSRV changed to 1.60 because of 'version-sink' crate.

TODO:

• update vpn example
• improve docs
• async integration tests
• more examples