Log message:
devel: Remove SHA1 hashes for distfiles

Log message:
Import auto-update-0.1.6

A common problem is the desire to have an action run at a scheduled
interval, but only if it is needed. For example, instead of having
every web request result in a new getCurrentTime call, we’d like to
have a single worker thread run every second, updating an
IORef. However, if the request frequency is less than once per second,
this is a pessimization, and worse, kills idle GC.

This library allows you to define actions which will either be
performed by a dedicated thread or, in times of low volume, will be
executed by the calling thread.