callback - Fixed type-deduction when inheritance is involved

The type deduction for the callback constructors was to strict and
prevented implicit casts for the context pointer stored internally.

As noted by @pan-, relaxing the contraints on the templated
parameters allows C++ to correctly infer implicit casts such as
conversions between child and parent classes when inheritance is
involved.

As an additional benefit, this may help the user experience by
defering invalid type errors to when the types are expanded,
limiting the number of error messages presented to users.

Author: geky