A FileMonitor is an abstraction for monitoring the status of files,
as well as changes in an in-memory value. Files can be tracked by file
modification time, or mod time plus content. We can also track files
that are expected not to exist (to help implement search paths). We can
also have file globs.

The abstraction is useful for re-running actions when input files or
values change. This pattern is captured by the Rebuild monad.

This adds a dependency on the hashable package (used by
unordered-containers). If this is a problem we can extract just the hash
function we need.

This is not used yet, so there's a temporary import just to make sure it
gets compiled.