Clean up "directory" package

[gfr10598]

Now that things have been re-organized it looks like "directory" is an increasingly bad name. If you called the directory timeannotator, then you could have the struct TimeAnnotator and then the functions Build and Merge. Build would still take a list of annotators, but Merge could have the type Merge(timeannotators ...TimeAnnotator) TimeAnnotator which makes things all cohere a little better in this module - every exported function then either makes a TimeAnnotator or takes a TimeAnnotator or both.

As a final suggestion, now that Directory only has one element why not make Directory (or TimeAnnotator if you decide to accept my earlier suggestion) a type alias for []annotator ?