[k2] implement async stack

[astrophysik]

General

This PR adds support for collecting async stack traces in runtime-light. Additionally, the implementation of debug_backtrace that uses coroutine traces is also added

Problem

Basic usage of backtrace with c++20 coroutines doesn't give enough information about execution. Common traces look like:

* nested_coroutine
* std::coroutine_handle<>::resume
* coroutine_handler_callee
* ...

Such traces don't show from which coroutine the coroutine requesting the stack trace was called. Moreover, at runtime, there should be the ability to reconstruct the sequence of coroutine calls not only for debugging purposes but also for language features, such as debug_backtrace, Exception::getTrace.

Solution

The solution is based on a similar approach from folly. InstanceState contains async_stack_root, which acts as the equivalent of the rbp register for the coroutine stack. To maintain a pointer to the top coroutine frame in async_stack_root, awaitables are used async_stack_watcher_t. High-level view can be found in the file async-stack.h.

Result

With the help of the asynchronous stack, it is possible to obtain a correct trace of coroutines, for example:

* nested_coroutine
* std::coroutine_handle<>::resume
* coroutine_func2
* coroutine_func1
* ...

Important aspects

• User code in KPHP is executed within forks. The top of the trace obtained from a fork will be the first function, which was the root of the fork

[apolyakov]

Great job!

[apolyakov]

LGTM!