move the functions from rmw.h into multiple separate files

[reed-lau]

rmw/rmw/include/rmw/event.h

Line 16 in 28f9185

#define RMW__EVENT_H_

here the event-related function is declared in a isolated event.h header file, but it is not included in the rmw.h.
when we implement a rmw-xxx, we have to include another rmw/event.h, why the event is so special.

[dirk-thomas]

I would rather ask the opposite question: why should everything be declared in a single header? The fact that this single header contains numerous unrelated function declarations and is 1000+ lines long would instead encourage me to split it up into separate files - each containing only functions of a specific category (e.g. pub/sub in one, services in another, etc.).

[reed-lau]

each containing only functions of a specific category

that sounds great!
I just mean, event is not special enough.