Move type/trait aliases to futures-util

[taiki-e]

This moves type aliases (BoxFuture, LocalBoxFuture, BoxStream, LocalBoxStream) and Trait aliases (TryFuture, TryStream) to futures-util.

part of #2207

• Type aliases (BoxFuture, LocalBoxFuture, BoxStream, LocalBoxStream)
  • Move to futures-util.
  • They are just type aliases of Pin<Box>ed future/stream.
• Trait aliases (TryFuture, TryStream)
  • Move to futures-util.
  • They are just trait aliases of future/stream that return Result, but the real trait aliases are not yet stable, so they have some limitations. See #2200 for one of the limitations.

~~This PR is marked as a draft because based on #2335. See the second commit for actual changes in this PR.~~

[taiki-e]

Oh, a lot of conflicts...