fix Cofree

1. remove unnecessary parameter
2. `object Cofree` should be extends `CofreeInstances`
Remove Endo#fix; add fix utility for functions, and test that it can be used.
Merge pull request #409 from xuwei-k/request/fix-cofree

fix Cofree
Merge pull request #410 from S11001001/topic/function-fix

Replace Endo#fix with a utility function