This was used in the past for reexporting the macros from drogue-device,
which is no longer using it.
Also, it is a pain to support, so we don't want it.
The normal `spawn()` methods can be called directly by the user, with arbitrary hand-implemented futures.
We can't enforce they're only called with `async fn` futures. Therefore, make these
require `F: Send`, and add a "private" one only for use in the macro, which can enforce it.
Fixes a new opaque type error in the task macro.
Full error is "opaque type's hidden type cannot be another opaque type from the same scope".
This got disallwed by the lazy-TAIT PR: https://github.com/rust-lang/rust/pull/94081
Sadly there's now some weird type inference errors with pre-lazy-TAIT
nightlies, so support for those is dropped.
