Fix a few things from cargo clippy

Some other comments and tidying, move lengths to constants

The Option's contents won't ever get zeroed,
instead we make a copy (or use a reference) and set Option to
None, forcing the drop.

Add license text for Embassy code

embassy picow demo fails to compile probably due to recent async
changes.

switch to custom branch for salty

Remove TCP timeout

Now successfully connects to std embassy demo

Removed Behaviour argument to server constructors

Also add some comments for potential channel changes

Added a generic poll_inner() to embassy_sunset
Added ChannelEOF error return type instead of None for EOF
Added app_done for channel handling code

Remove some unneeded trait functions in std demo

Now is common between CliBehaviour and ServBehaviour

Thanks to bruh![moment] on Discord for details how to make it work

Improve the roundtrip pubkey auth test

std and picow embassy server demos work, no channels yet