[k2] add await set primitive

[astrophysik]

General

This pull request adds an equivalent of JoinSet from rust-tokio to runtime-light. This primitive allows coroutines to be dynamically added to it and waits for their completion.

Example:

kphp::coro::await_set<int64_t> await_set{};
for (int i = 0; i < 5; ++i) {
    await_set.push(coroutine_call());
}

while (!await_set.empty()) {
    auto res = co_await await_set.next();
    // process result here
}

The primitive supports multiple awaiters. All awaiters are stored in a linked list.

In the constructor, you can also set the waiting policy. Two options are available - suspend_on_empty and resume_on_empty.
They affect two aspects:

1. What happens when waiting begins on an empty set
2. What happens when the set becomes empty during waiting.

Details

The implementation of the primitive is similar to the implementations of when_any and when_all. The awaitables passed to push are wrapped in an await_set_task, in the final_suspend of which the result is stored. Awaiters are linked to the list of results through the await_broker class, which maintains a singly linked list of completed tasks and a doubly linked list of awaiters. As a remark, I would like to note that awaiters and ready tasks are stored in the lists as LIFO.

Motivation

This is the first step towards implementing wait_queue builtins in the new runtime. Here's how a wait_queue can be approximately implemented using a primitive.

inline kphp::coro::shared_task<int64_t> fork_function() {  
  co_await kphp::coro::io_scheduler::get().yield_for(10ms);  
  co_return 42;  
}  
  
  
inline kphp::coro::task<kphp::coro::shared_task<>> helper_task(kphp::coro::shared_task<int64_t> shared_task) {  
  co_await shared_task.when_ready();  
  co_return shared_task;  
}

auto shared_task = fork_function();
kphp::coro::await_set<kphp::coro::shared_task<>> wait_queue;
wait_queue.push(helper_task(shared_task))
auto erased_task = co_await wait_queue.next();
auto task = static_cast<kphp::coro::shared_task<int64_t>>(std::move(erased_task));  
auto res = co_await task;  
f$var_dump(res);

[apolyakov]

Summary after discussion:

1. There should be a single policy of how await_set works
2. abort_all should not prevent awaiters from awaiting on the same await_set
3. await_set should be movable
4. kphp::coro should only contain await_set, no other symbols are required in public API

[apolyakov]

LGTM