Created CorrespondenceTests
Fixed LinkedList.keys implementation
Added tests for List.shorterThan and List.longerThan
Made ListTests satisfy CorrespondenceTests
Removed special case in LinkedList.keys as pointed out by @lucaswerkmeister
Completed tests for Correspondence - all methods tested
Moved implementation of Correspondence.keys up from LinkedList to MutableList as it seems an appropriate implementation for all lists
Created IterableTests. Pulled some tests from ListTests to IterableTests
Made SetTests satisfy IterableTests - fixed some tests
Overrode test methods for IterableTests in UnlinkedHashSetTest as UnlinkedHashSets do not need to obey insertion order
Fixed HashSet(unlinked).first and last which returned null for single-item sets
Overrode some IterableTests in TreeSetTest as TreeSets keep elements sorted in natural order
Implemented tests for all methods of Iterable. Tests which are hard to write without taking into consideration the order of elements were implemented by smaller sub-classes of IterableTests, namely NaturalOrderIterableTests, InsertionOrderIterableTests and HashOrderIterableTests
Created specialization of IterableTests and its order-specific variants for Iterables which allow null elements
Modified all Iterable tests (except PriorityQueue) to use the new IterableTests and its specializations. Removed tests which would become duplicates.
Merge pull request #238 from renatoathaydes/more-list-tests

More list tests
use ArrayList instead of SequenceBuilder
work around a compile problem that only occurs in the IDE
fix after change to typechecker
small changes to API of MutableList