consistently create thread and task when entering component instance

[dicej]

Previously, we weren't creating a new thread or task in all cases when entering a component instance, even when component model async features were enabled. In particular, entering an instance via a sync-to-sync, guest-to-guest adapter, via Linker::instantiate[_async], or via [Typed]Func::call all skipped creating a thread or task, leading to panics and/or instance mismatches in certain cases.

This commit addresses all those cases and also adds assertions to all CM async intrinsics to verify that the caller instance matches the most-recently-pushed task. Note that we still skip pushing and popping threads and tasks if no CM async features are enabled in the Config.

In order to populate the GuestTask::instance field for tasks created as part of Linker::instantiate[_async] calls, I had to add a RuntimeComponentInstanceIndex field to GlobalInitializer::InstantiateModule and friends so it would be available when needed.

While testing this, I uncovered and fixed a couple of related issues:

• We weren't checking the may_leave flag when guest-to-guest calling a resource destructor
• We weren't checking whether a subtask was ready to delete (e.g. that no threads were still running) before attempting to delete it while deleting its supertask

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[dicej]

I've rebased this onto main and addressed the feedback so far locally. Once WebAssembly/component-model#601 is merged, I'll push my updates.

[alexcrichton]

Two other possible spots worth scrutinizing:

• cabi_realloc invocations - probably don't need special treatment? Unsure.
• ResourceAny::resource_drop - I think this'll need task-related treatment?

Although given the litany of entrypoints into wasm I'm finding it really hard to keep them all in sync, so it's probably also worth refactoring in theory to only have one entrypoint somewhere

[dicej]

* `cabi_realloc` invocations - probably don't need special treatment? Unsure.

As with post-return functions, we don't allow cabi_realloc functions to leave the instance.

* `ResourceAny::resource_drop` - I think this'll need task-related treatment?

Yup, I'll address that and add a test.

Although given the litany of entrypoints into wasm I'm finding it really hard to keep them all in sync, so it's probably also worth refactoring in theory to only have one entrypoint somewhere

I'll open an issue for that.

[alexcrichton]

Right yeah cabi_realloc can't leave, but do we have tests for that?