asio: 1.10.8 -> 1.12.0

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 1.12.0 with grep in /nix/store/1js10q8jgqmpk6ashz63am7f9xl37kvs-asio-1.12.0
- directory tree listing: https://gist.github.com/834ad2743ce2f56612f8ba9ff9a4e5b5