[communication] Revive Asynchronous Multi-Node Bus (AMNB) protocol

[salkinium]

A flexible multi-node bus using p-persistent CSMA/CD over a shared wired medium.
This can work with using UART with differential signalling (typically CAN transceivers for automatic direction control) or without for short distances with TX in Open-Drain mode with an external pull-up, for some STM32 even just using a single pin via the built-in half-duplex mode for UART.

Consider this a low-cost alternative to I2C, with the goal to be more stable, more flexible and faster.

[salkinium]

Design goals:

• important information (address, command, type) in the header first, so I can discard the message if it's not for me.
• very little overhead for small messages, here CRC8 can protect (max 256 bits => 32 bytes incl. header).
• support large payloads via CRC16 (max 2^16 bits => 8kB).
• small payload header is compatible with large payload header, requiring only the first 4 Bytes to decide if its a small or large payload, and max 32 Bytes to decide whether the heade is valid (because small payload is part of the header CRC8).
• to have the payload in a continous memory to be able to reinterpret_cast the payload on-device to whatever the type is, to avoid explicit serialization just like in XPCC. This requires 4-bytes alignment of both payloads.
• Simply "filling" the values on the wire into the packed message struct is enough. This involves a choosing a layout that avoids unaligned access for Cortex-M0, requiring 2-bytes alignment for uint16_t and 4-bytes alignment for uint32_t pointers.
• Option to convert payload into shared_ptr, which requires only copying small payloads and because the large payloads are already allocated on the heap anyways.
• Delegating memory management to the application or at least the node, so the protocol stack doesn't require any buffers and only operates on messages owned somewhere else.

The C++ api is still WIP and currently untested.

@dergraaf You're a protocol wizzard, would you mind giving some feedback?
@rleh @chris-durand @se-bi I'm using this for the ELVA semaphores, but perhaps this interesting for RCA use-cases too?

[salkinium]

Things that really bug me: There seems to be no way in the UART hardware to indicate whether a symbol is currently being received. All the RX flags are only set after the full symbol has been received. This feature could turn the "Collision Detection" into a "Collision Avoidance", which would be a nice stretch goal.

[salkinium]

I'll add documentation and AVR examples at a later stage.