Expose a C-API for legacy bare-metal systems

[florianhumblot]

Is there a way to condense the smoltcp API so that it could be exported as a C library?

With the design goals stated in the readme, this library seems particularly well suited for embedded systems, and considering the mess that is LwIP, I think it could be interesting to use smoltcp (which is extensively tested and fuzzed) as the TCP/IP stack instead of LwIP or other C-based TCP/IP stacks.

Example use case:

• FreeRTOS running on a microcontroller
• Uses LwIP on a seperate task for networking, does a bunch of heap allocations within FreeRTOS