Correct increased memory usage

[szszszsz]

Correct increased memory usage by using references instead of the heapless buffers, and switching to serde_bytes for the serialization.

[sosthene-nitrokey]

LGTM overall, but it appears that not all commands use references (for example CommandOpenPGPInfoResponse does not).

[szszszsz]

it appears that not all commands use references (for example CommandOpenPGPInfoResponse does not).

That's correct - I was prioritizing the failing command first. Will finish that in the next steps.

[sosthene-nitrokey]

Protip to check the size of the types: cargo +nightly rustc --release -- -Zprint-type-sizes |& tee /tmp/type-sizes

Then you can peek into type-sizes to see where are the issues. I then used rg '\d{4} bytes' /tmp/type-sizes to see sizes larger than 1000 bytes.

[szszszsz]

To add, these were informative for me too:

• arm-none-eabi-objdump -d runner-nrf52-nk3am.elf | c++filt
• nm runner-nrf52-nk3am.elf -SCln

I think, that cargo bloat is not the best approximation for the embedded firmware case

[szszszsz]

Connected: Nitrokey/nitrokey-3-firmware#312

[szszszsz]

There were no heavy changes since the last review, only some clean up. I plan to merge the current state as-is, in a squashed commit.