Avoid async method delegate allocation

Avoid async method delegate allocation

Previously when a task-returning async method would yield for the first time, there would be four allocations: the task, the state machine object boxed to the heap, a context "runner" object, and a delegate that points to the boxed state machine's MoveNext method.  A recent PR changed this to avoid the separate box object and the runner, but that still left the task and the delegate.

This PR avoids the delegate as well in a common case.  For async methods that only ever await Task/Task`1, that aren't using a custom sync context/scheduler, and for which tracing isn't enabled, we know the inner workings of both the builder and the awaiter and can thus bypass the awaiter's pattern APIs; instead of creating the delegate that gets passed to the awaiter and then stored in the wrapped task's continuation slot/list, we can instead just store the boxed state machine directly in the slot/list.