stm-orelse-io

While tinkering on a project, I frequently found myself wanting to wait for an IO operation unless something more important came up. The Control.Concurrent.STM.OrElseIO module is meant to reduce the boilerplate needed when choosing between IO actions and STM operations. You must use the threaded runtime with this package. TODO: Kill off IO threads whose return values will never be needed. Possibly rename runOrElse and friends to orElseIO or similar.