Allow ?Sized types in some methods and structs

[taiki-e]

No description provided.

[cramertj]

I'm not sure if we want to reveal that this is allocated separately-- I had imagined at some point we might want to switch to using e.g. parking_lot's Mutex type which prevents an allocation here. I don't feel super strongly about it, though, since futures-util should be okay to make breaking changes to in the future. Your choice :)

[taiki-e]

@cramertj
I'm sorry, I'm not sure what is the problem.

• In this pr's implementation, if T is !Sized, then Mutex itself becomes !Sized.

  use futures::lock::Mutex;
  use std::sync::Arc;
  fn foo() -> Arc<Foo<dyn Fn()>> {
      let m = Foo::new(|| {});
      Arc::new(m) as Arc<_>
  }
  fn bar() -> Foo<dyn Fn()> { //~ ERROR the size for values of type `(dyn std::ops::Fn() + 'static)` cannot be known at compilation time
      let m = Foo::new(|| {});
      m as _
  }

• parking_lot's Mutex allows T: ?Sized, and if T is !Sized, the behavior is the same as this pr's implementation: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a4ecc3eecc9fa5cdce41bebd5f97cf5e.

[cramertj]

Ah, I see-- I was thinking that the Mutex here was still Sized even if T: ?Sized.

[cramertj]

Nice! One question re Mutex but otherwise the rest of the changes LGTM.