en, a C library will allow setting some kind
of user data, which will then be passed back when the callback is invoked by the library. One could put a
<code class="mendoza-code">JanetFunction *</code> into that user data slot and have a common "trampoline" native function
that can be a sort of universal callback that would call that userdata parameter it received as a JanetFunction.
While this is far from general, it is effective in many cases, and so the <code class="mendoza-code">ffi/module</code> provide