BLE "just works" pairing dos not protect against active MITM,
so we need to handle this ourselves. Use nacl/libsodium
crypto_box to encrypt and authenticate the wifi password
this also adds a step to the interaction between client and device:
after writing the password, we now write a "true" to the "connecting"
characteristic to kick off the actual conection attempt.
I would rather like to be able to write tests for the complicated
bits, but using "cargo test" with the esp32 tolchain/libraries
is beyond my skill level if it's even possible. Hence we now have
an ecu-portable crate for stuff that can be built using plain old Rust
(e.g. x86_64 with std) and an ecu-esp32 crate for the rest of it
dan