utilities/file_location_listing#64: Consume headings index when searching

utilities/file_location_listing#60 implemented collection and indexing of page headings.

However, that index is not currently consumed (because I wanted to be able to see how the resulting indexes looked first).

Assuming indexes look OK, we need to implement the ability to load the index for use when searching.

Conceptually, it should probably work much the same way as the tags index does - precalculating matches for use when iterating the main index.