v0.18.1

Fixed: the server loop no longer starves other async tasks' socket I/O (the drain now suspends on its whole socket set via await-readable-any instead of busy-spinning socket-select), and http-server-stop now actually stops a running serve loop (the stop signal lives in a shared one-slot box carried by reference through the derived records; stopping an idle server completes in ~100ms). Requires sigil 0.17.10+ for await-readable-any; the sigil: requirement is bumped to ^0.17.10.