Re-export some items for select! macro

[taiki-e]

Fixes #1404

select! macro will use the item in futures::{future, task} if imported from futures and the item in futures_util::{future, task} if imported from futures_util. This error was caused by some items not exported in futures_util.

This PR re-exports some items to futures_util, and adds a test of the case where select! macro is imported from futures_util.

[taiki-e]

Merging #1407 will fix build failure.

[taiki-e]

FYI: 1407 + this PR
(This is the build result of the code which branches from this branch and did git rebase.)

[cramertj]

Thanks for the fixes!