C++/C interop support

[itrofimow]

Hi!

I work on a decently large C/C++ code base, and finding creative ways to do some of the things showcased in this repository is a big part of my day-to-day job. Seeing that it's possible to achieve them in a memory-safe way fills me with joy, and i would really like to introduce Rust at my workplace, so could you please export some of the functionality in a C-consumable way for easier adoption?

I'm mostly interested in null_mut for now.

[5GameMaker]

#20