Auto merge of #51944 - MajorBreakfast:generic-future-obj, r=cramertj

Auto merge of #51944 - MajorBreakfast:generic-future-obj, r=cramertj

Make custom trait object for `Future` generic

- `TaskObj` -> `FutureObj<'static, ()>`
- The `impl From<...> for FutureObj<'a, T>` impls are impossible because of the type parameter `T`. The impl has to live in libstd, but `FutureObj<'a, T>` is from libcore. Therefore `Into<FutureObj<'a, T>>` was implemented instead. Edit: This didn‘t compile without warnings. I am now using non-generic Form impls.

See https://github.com/rust-lang-nursery/futures-rs/issues/1058

r? @cramertj

Edit: Added lifetime