Support #f as nullptr in (* ftype)

[xaengceilbiths]

It is painful to auto-generate and use bindings for a library with 100 entries, 5 args per entry with some default (* ftype) value NULL. Therefore it is a good idea to support #f as nullptr in (* ftype) args, just like #f for nullptr in strings. Maybe my implementation will help: https://github.com/xaengceilbiths/ChezScheme I wrote some tests: https://gist.github.com/xaengceilbiths/434ec8e654fd7bd6b6ba5ab5381b939e

[amirouche]

Here is the relevant commit https://github.com/xaengceilbiths/ChezScheme/commit/3ce57d631091057d2b80be659525cfd1d3ab49a8