Remove some unused variance, with a little remaining breakage in Free.
Backtrack a lot; scalac tricked me.
Avoid illegal variance in bitraverseImpls.
Avoid illegal variance in traverseImpls.
Remove more unused variance.
fix MapTest
Merge pull request #343 from xuwei-k/fix-MapTest

fix MapTest
Specify scala syntax for OptionT example
Make 'Type Class Hierarchy' example valid
Merge pull request #344 from bmjames/scalaz-seven

Minor Readme fixes
Merge pull request #328 from S11001001/topic/hk-ev-variance

Don't break SI-2066 (non-trans)