Callbacks covariance and contravariance

Callbacks overriding should work the same way as methods usually work

[ ] Changing meta may be impossible, same as in #55
[ ] Input parameters can be less specific (callback-specific rules may apply)
[ ] Return type may be more specific (callback-specific rules may apply)
[ ] Callbacks should be validated in scope of all classes which define or change them

orisai / object-mapper