typesafe_cb: simplify, preserve namespace.

typesafe_cb: simplify, preserve namespace.

Get rid of many variants, which were just confusing for most people.
Keep typesafe_cb(), typesafe_cb_preargs() and typesafe_cb_postarts(),
and rework cast_if_type() into typesafe_cb_cast() so we stay in our
namespace.

I should have done this as soon as I discovered the limitation that
the types have to be defined if I want const-taking callbacks.