Collectors: Add base class

A Collector is the base class of anything that collects a set of
arbitrary items. It provides mainly iterators.
tests: Add collecting package
collectors: Add Collector tests
Collectors: Add FileCollector
Collectors: Add FileCollector tests
Collectors: Add FilterCollector
Collectors: Add FilterCollector tests
coverage: Exclude any temporary files

We sometimes use temporary code files for tests. Branch and statement
coverage for these doesnt make sense and is of no use at all.