add cco macro, block style coroutine definition

[liigo]

to replace cco_begin(co) && cco_end().

int task1(struct context_t *ctx) {
    cco(ctx) {
        puts("1");
        cco_yield(1);
        puts("2");
        cco_yield(2);
        puts("3");
        cco_yield(3);
    }
    puts("ending");
    return -99;
}

[tylov]

@liigo, this is just brilliant! Thank you! There are only a few things to fix:

1. I think the name cco() is very short for a public library macro, and not very descriptive. Maybe cco_routine() would be clearer for readers who are not closely familiar with STC (yet :). Or do you have a better suggestion?
2. Both cco_begin() and cco_end() macros should be removed.
3. That means replace cco_begin(x); ... cco_end() with cco_routine(x) { ... } in all examples so they compile. (examples will be reorganised/filtered at some point).
4. In cco_await_2, replace return ret; with {return ret; goto _begin;} to avoid unused label warning (as you did in cco_yield).
5. I can update the documentation later.

[tylov]

I decided to accept your PR as it is, and I will do some cleanup myself later, i.e. rename cco() to cco_routine().
Thank you again.