Add inplace callback to dynlibmod, improve example

Add inplace callback to dynlibmod, improve example

This adds the possibility to properly register inplace callbacks in the
dynamic library module. It works by creating a wrapper procedure that
is available to the dynamic library and will call the given callback
through a whitelisted callback function.

The dynamic library example has already been improved to include
comments and some simple examples on allocating and deallocating memory
and registering callbacks.