Payload buffering and swapping

[jleni]

We should port the functionality that exist in buffering.c to rust.

What we are looking for:

• Two configurable (build time) buffer sizes for RAM and NVRAM
• as we receive payloads, they are collected in the RAM buffer
• once the RAM buffer is full, it will swap all the content and start collecting in NVRAM
• we should be able to mock the NVRAM when a Ledger backend is not used

This buffering object should be able to provide a non-mutable slice to other application objects.

References:

• nvram management in Rust that could be used as a reference

• Application example

• Let's add unit testing for this feature.

• We can also create a fake INS (in the 0xF0 .. 0xFF range) that should only be available in "test" mode that can be used to run tests in Zemu.

[ghost]

Looking at ledger's code we might need to provide PIC functionality first before being able to do this, unless we want to plug that in later afterwards, but I'm not entirely sure if that's feasible, with the emulator and all...

---

Ah but we have a rudimentary pic present in the bolos module...

[jleni]

Yes, that PIC function has been used by us already in other places. It works well

[ghost]

TODO: add Zemu test for INS_DEV_HASH