blocking now fails to compile on 1.47

[notgull]

See smol-rs/async-io#93. With the new release of once_cell, we no longer compile on 1.47.

cargo +1.47 check                          
warning: unused manifest key: package.rust-version
error: failed to download `once_cell v1.15.0`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/jtnunley/.cargo/registry/src/github.com-1ecc6299db9ec823/once_cell-1.15.0/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.

[notgull]

Closed by #30