Renaming single element map to reflect that its as well and Map.Entry instance.
Adding hashCode() and equals() implementations to Map types.
Merge branch 'master' into persistent-set
Adding Set specialisations up to 5 elements.
Changing PDBPersistentHashSet/TrieSet operations to operate with "equivalence" by default.

This is a shortcut that used for the moment to make it compatible with isEqual comparison.

This commit is an addition to the previous one to make it consistent.
Reusing specialised Set implementation wherever a Set view is demanded.