IList; safe, invariant cons list
operator tests; right-assoc append fix
removed Each instance
improved collectFirst, removed obsolete comment
Avoid syntax._ bindings in IList.

see https://github.com/scalaz/scalaz/blob/v7.0.5/doc/DeveloperGuide.md#using-type-classes
Some IList finality & improved inference.
Merge pull request #611 from tpolecat/ilist2

IList: safe, invariant cons list