Frantic day today, around 10h of productive work on improving the Index and moving it as part of the go-ap/filters module.

+1510/-11 lines of which 987 belong to tests.

Coverage is not entirely sufficient yet, because it's missing the checks for the top level Index.Add() and Index.Search() methods.

Another thing left to do is the persistence to disk.

The **reason** why I wanted to move the work I've done yesterday to this module is that instead of the custom client.SearchByX() functions, I wanted to retrofit the existing functionality already present in the filters module. Ah, also moving the bitmaps themselves to a semblance of generic types....