Fix build error on master

[taiki-e]

impl Valuable for Arc<[T]> is added in #13, but when that PR was opened, #12 had not yet been merged, so #13 did not handle targets that do not support atomic CAS.

As a result, the CI of the master branch is currently failing:

error[E0433]: failed to resolve: could not find `sync` in `alloc`
  --> valuable/src/listable.rs:73:26
   |
73 |     (T: Valuable) alloc::sync::Arc<[T]>,
   |                          ^^^^ could not find `sync` in `alloc`

https://github.com/tokio-rs/valuable/runs/2560992823?check_suite_focus=true