smol violates API guidelines' "C-STABLE"

[taiki-e]

https://rust-lang.github.io/api-guidelines/necessities.html#public-dependencies-of-a-stable-crate-are-stable-c-stable

Public dependencies of a stable crate are stable (C-STABLE)

A crate cannot be stable (>=1.0.0) without all of its public dependencies being stable.

[taiki-e]

We can resolve this in smol 3.0 at the latest as there can be major changes in the std stream trait and std async io trait.

[taiki-e]

It will take at least a year to resolve this, and there is little that can be done on the smol side, so I will close this as blocked-closed.