Implemented sub-tree check
Implemented gob based index writing

This openes the dor to the verification step, which will challenge
the architecture of the program.
I suppose it's a good idea to refactor now and split things up into
more file.
Improved index output file name
Minor seal test cleanup
Split everything up into multiple files, matching the algorithm
Moved utility types into their own package.

Will have to check if these can be in the top-level, godi, as well
reorganized package names.

api -> godi

Now I believe to have understood how this works