Everything now builds on stable. Hopefully nightly doesn't break
anything (that's what crater's for, right?)

Uses ~120 bytes more stack. Can be replaced once PR is merged
https://github.com/dalek-cryptography/curve25519-dalek/pull/556

RSA had a few changes in 0.8->0.9 that needed handling

ssh-key's algorithm name is no longer 'static.

The server should expect a banner line straight away, avoids dangling
connections from port scanners etc

They still need to be public for some reason, even
though they don't seem to be in input or output arguments.

Instead it can just be a generic parameter for methods that take a Behaviour<C, S>
argument, is tidier.

Some unnecessary members are no longer exported, and some docs improved.

This updates to latest git embassy deps (soon to be published on
crates.io).

The two nightly rust features are no longer required. Async Fn in Traits
has been merged.

Type Alias Impl Trait isn't in stable yet, so uses of singleton!() need
to be replaced with more cumbersome StaticCell. Eventually that can be
reverted.

There are also some small API changes to embassy and embedded-io crates
that needed fixing.

https://github.com/rust-embedded/cortex-m/discussions/503
-z is 229kB, -s is 248KB text, so worth keeping -z

Only KEX packets are allowed during KEX.
First packet must be kexinit when strict-kex
is negotiated

Includes updating some embassy versions, takepipe needed restructuring
to use split(). Takepipe isn't tested yet

Required updating embedded-io-async too

Also update to latest cyw firmware

Some picow menu improvements

Bump embassy version, embassy-net-tuntap is now its own crate
Also fix some warnings

Fix path for "size" of target in ci

Add sshwire::ssh_write_vec() helper

Wasn't being saved to flash before

Untested

A few other bits of tidying too