Implement postings lists using a singly-linked list

Postings lists used to be implemented as TreeMaps, which is sort of cheating and don't map easily on to disk (which is a necessity in a big search engine). Disjunctive merging is now O(n) using a zip-merge algorithm.

Test methods are also renamed as shouldDoBla.
Search by conjunctive query

Implemented conjunctive query in linear time. Also added some extra tests for the postings list and streamlined testing. Package adt renamed to datastructures.