Ugly generic parameters and longer code, but it should be reusable.

A new GlobalState struct is passed around, with sessions taking a local
copy of SSHConfig.

- Can switch to usb serial mode, various other improvements to usb
  serial, can exit cleanly.
- Wifi config code written, but is unsted because it's only accessible
  by wifi...
- hostkeys() behaviour now returns a heapless::Vec

Previously it only woke when a read/write occurred.

ctrl-w and ctrl-u to erase, ctrl-c to cancel

Currently doesn't seem useful, seems like it would need more context
if it were being used.

Use READ_SIZE to size the serial read buffer that is
sent directly to ssh

Some dep crates are still always using defmt though, needs fixing.

This should let us ignore ZLP problems.
Perhaps performance could be improved later.

Allows ssh serial@picow to attach to the usb serial port

- takepipe needs better mutex handling, currently take() only
completes when the current user performs a read/write.

- Updated toolchain and cyw43

- Added context to demo Shell.

More async auth methods will probably follow

It's slow on the rp2040. Instead we should add entropy inside
the ed25519 signature generation.

Perhaps should have a different name, is mostly internal conveniences

A cutdown std program may only want ed25519

Is slightly more consistent

It isn't used and didn't seem to work with panic=abort experiments

- ServBehaviour::hostkeys() now returns a slice of references
  since we can't clone SignKeys and don't want to move
  them out of Config. May need rethinking.

- embassy demos common code is now a proper crate

- Updated various embassy deps

Fixes build failures. Toolchain is updated, but we can't upgrade to
latest nightly because it fails with an ICE, possibly
https://github.com/rust-lang/rust/issues/110206

thread 'rustc' panicked at 'DefId(35:311 ~ embedded_io[99f8]::asynch::Write::write::{opaque#0}) does not have a "def_span"', compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs:205:1

Also fix pubkey packet test and fix some doc formatting

Remove some unneeded messages, don't panic on TCP socket errors

Fixes hanging at the end of "git push"

This allows it to work when use for eg git

Requires alloc. Only tested with std.

Some other RAM reductions

We don't need OwnedSig state any more (we send straight away), simplify
packet creation.

sunsetc host -s sftp

would fail before. argh has a "greedy" attribute to handle that