zflecs: Added zflecs_init and zflecs_fini.

[Srekel]

Hi, this PR is very open to feedback but I think the core of it is reasonable. It moves a few lines out of zflecs.init and zflecs.fini to two new functions, zflecs.zflecs_init and zflecs.zflecs_fini.

The purpose/result is twofold:

• Take a step towards zflecs supporting more than one world by moving away things that doesn't strictly have to do with world lifetimes from the world lifetime functions.
• Proper setup of the abort function. Previously, in the order it was done, meant that it would in fact get overridden by flecs to the default handler and was therefore in fact not used at all. Now if flecs asserts while debugging, we will end up at the @breakpoint line.

Here's what my game code looks like:

    ecs.zflecs_init();
    defer ecs.zflecs_fini();
    var ecsu_world = ecsu.World.init();
    defer ecsu_world.deinit();

[hazeycode]

Hey @Srekel are you still wanting to merge this?

I'm not very familiar with flecs but this refactor looks good to me. Although I wonder if the init and fini should be moved to World.init and World.fini and then have zflecs_init and zflecs_fini take the names init and fini, or am I reading this wrong?

[Srekel]

I'll need to get back to you on this a bit later once I've double-checked the changes :)