[native-mt] Fix ObjC autorelease object leaks with native-mt dispatchers

[andersio]

Resolves #2322

While 1.4.30-M1 fixed Worker.execute() leaking ObjC autoreleasing objects when running native code on a K/N Worker thread (KT-42822), it does not address the issue at KoltinX Coroutines framework level.

More specifically, when tasks on an EventLoop call into native code, any autoreleasing object — especially many ObjC method return values — is effectively leaked ^[1]. This is because the first available autoreleasepool in the call stack is in workerMain(), and it wraps around a most likely never-returning ^[2] runEventLoop().

This PR introduced a failing test case of this bug, and fixed it by wrapping all Runnable.run() calls in EventLoopImplBase with an autoreleasepool (or nothing for non-Darwin targets). This is equivalent to the DISPATCH_AUTORELEASE_FREQUENCY_WORK_ITEM^[3] in GCD.

An alternative to the per-item wrapping is integrating pool push/pop into runEventLoop(), with heruistics based on the park time returned by processNextEvent(). Not familiar enough about the particulars to try this out though.

^[1] ... unless an autoreleasepool is declared in user code explicitly, which is a rare practice except for memory optimization.
^[2] Dispatchers.Default, for example.
^[3] ... which is not the default behaviour. GCD threads inherit autorelease pool management from NSRunLoop, which recreates pools for each runloop iteration.

[elizarov]

Looks great to me! Thanks a lot.

[elizarov]

Does not compile:
Expected function 'platformAutoreleasePool' has no actual declaration in module <kotlinx-coroutines-core> for JVM
It needs to be implemented as a NOP both for JVM and JS.

[qwwdfsad]

Great job! 👍 Thanks for a detailed write-up and isolated unit tests, it really made the review process easy and pleasant