callback - Reordered optional argument to static C function

Before:
Callback<void()> a = callback(obj, member)
Callback<void()> b = callback(context, function)

After:
Callback<void()> a = callback(obj, member)
Callback<void()> b = callback(function, context)

This ordering is more intuitive based on feedback from users. This order
was initially considered but proved problematic when integrated with
other variable arguments in attach functions.

With `callback` as a separate convenience function, this style
no longer presents a problem.

Author: geky