Change FutureObj to use a *mut dyn Future fat pointer

[Nemo157]

r? @cramertj

[yoshuawuyts]

@Nemo157 looks like this has a merge conflict.

[Nemo157]

Conflict is probably just around the feature changes, I'll fix it soon. But this is also blocked on a new nightly being released with rust-lang/rust#60165 included.

[cramertj]

@Nemo157 Pin is repr(transparent), so it'd be sound to add those conversions in an unsafe manner to futures-rs. I'd personally prefer to do that now rather than to take a dependency on a new unstable API.

[Nemo157]

👍 sure, I'll push an update switching back to doing the conversion manually.