ObjFW  Check-in [d16ad96cbd]

The target / selector approach had several drawbacks:

* It was inconvenient to use, as for every read or write, a target,
selector and context would need to be specified.
* It lacked any kind of type-safety and would not even warn about using
a callback method with the wrong number of parameters.
* It encouraged using a different callback method for each read or
write call, which results in code that is hard to follow and also
slower (as it needs to recreate the async operation with a new
callback every time).