Remove dependency on futures-util

[yoshuawuyts]

Right now we're pulling in quite a few dependencies through futures-util-preview. The only feature we're currently using that's included in it is AtomicWaker. I'm wondering if we may perhaps be able to drop this dep somehow.

cc/ @stjepang thoughts?

As far as I can tell, only this is pulled in by futures-timer:

    ├── futures-timer v1.0.2
    │   ├── futures-core-preview v0.3.0-alpha.19 (*)
    │   ├── futures-util-preview v0.3.0-alpha.19
    │   │   ├── futures-core-preview v0.3.0-alpha.19 (*)
    │   │   ├── pin-utils v0.1.0-alpha.4 (*)
    │   │   └── slab v0.4.2
    │   └── pin-utils v0.1.0-alpha.4 (*)

So the futures-util-preview is not really adding much.

If need be, AtomicWaker is relatively simple and could be copied over into this repository.

[taiki-e]

Right now we're pulling in quite a few dependencies through futures-util-preview.

FYI: I've made many dependencies of futures-util optional in rust-lang/futures-rs#1709.

[yoshuawuyts]

@stjepang in terms of nested dependencies it isn't much; but we're essentially compiling all combinators twice right now: https://docs.rs/futures-util-preview/0.3.0-alpha.19/futures_util/

This seems like it would add up, and being able to shave it off seems relatively useful (:

Let's try moving AtomicWaker here then. :)