Remove need for `ResumeTy` / `get_context` in async generators

[Swatinem]

#104321 changed async blocks/functions to not go through from_generator / GenFuture, but made those generators implement Future directly.

Though inside those special generators, it is still necessary to hide the &Context<'_> poll argument behind an unsafe ResumeTy wrapper which removes any lifetime constraints.

It should be possible to remove that type, and the unsafe get_context function which turns it back into a &Context<'_> to poll child futures.

With #104321 the compiler already "cheats" and magically turns the &Context<'_> of the Future::poll call into an internal Generator::resume(ResumeTy).