Introduce new CompostCtx struct that is passed into all the handlers

[blazra]

Allocator is now passed as part of the ctx. ctx also contains the rx and tx messages and user_ctx - a void pointer passed by the user to compost_msg_process()

Documentation needs to be changed.

PowerPC tests to be done.

[moucha19-st]

The idea and implementation is solid, this change will cover a lot of different use cases in the future.

One small thing though, the test binary compiles with warnings due to unused ctx argument:

protocol_impl.c: In function ‘trigger_notification_handler’:
protocol_impl.c:68:54: warning: unused parameter ‘ctx’ [-Wunused-parameter]
   68 | void trigger_notification_handler(struct CompostCtx *ctx, uint16_t msg_id)
      |                                   ~~~~~~~~~~~~~~~~~~~^~~

I think this is the only drawback, that handler will always have this parameter but user won't necessarily need it every time. But I think it's fine, compared to some more complicated solutions, like having it optionally for only some RPC's.
However, you should still get rid of the warnings in the test handlers :)

[blazraST]

Yeah, the unused warning is annoying. I don't think there is a way how to silence it in C11. User will have to do the cast to void trick to silence the unused warning. I will do it in the test.

[blazraST]

PowerPC tests are passing. No more warnings. Ready to merge.