pw_async2: Use Poll<void> instead of Poll<ReadyType> by default

Instead of requiring futures to use `ReadyType` instead of `void`, have
`Poll` default to `void`. Specialize `Poll<void>` to have `value()`
return `ReadyType` to simplify template code. Offer `FutureValue` that
accesses `Future::value_type`, but replacing `void` with `ReadyType`.

Change-Id: Ib4568d7492a3b0d041be7b0bec01f33d74adbe12
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/369832
Reviewed-by: Alexei Frolov <frolv@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
Copybara-Verified: Copybara Prod <copybara-worker-blackhole@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>