With the advent of the Component Model concurrency ABI and it's `task.return`
intrinsic, post-return functions have been informally deprecated and are
expected to be removed for WASI 1.0 and the corresponding stable edition of the
Component Model.  Consequently, it does not make sense anymore to require
embedders to explicitly call the post-return function after using
`[Typed]Func::call[_async]`.

As of this commit, `[Typed]Func::post_return[_async]` are no-ops.  Instead, the
post-return function is called automatically as part of
`[Typed]Func::call[_async]` if present, which is how
`[Typed]Func::call_concurrent` has worked all along.  In addition, this commit
fixes and tests a couple of cases where the task and/or thread was being
disposed of before the post-return function was called.

Specifically, I've split the `invoke_post_return` test into multiple tests:

- using `TypedFunc::call`
- using `TypedFunc::call_async` with concurrency support enabled
- using `TypedFunc::call_async` with concurrency support disabled
- using `Func::call_async` with concurrency support disabled
- using `TypedFunc::call_concurrent`

This broke the MSVC build.

Now that post-return calls are handled internally without requiring explicit
action by the embedder, we can avoid unnecessary bookkeeping.