EventLoop::post() and the sync() wrapper around it did the same thing, so this merges them into a single sync() method. And also takes the callback as a kj::FunctionParam instead of a kj::Function: FunctionParam stores only a pointer to the caller's callable rather than taking ownership of it, so the callable no longer needs to be copyable or movable, and each call avoids a heap allocation.
This is safe because sync() does not return until the event loop thread has finished running the callback and cleared m_post_fn(now named m_sync_fn), so the pointer is never used after the callable goes out of scope.
