Support function pointers

[adetaylor]

[wmmc88]

Are all function pointers still not supported in autocxx?

[dougcooper]

As far as i can tell this is a limitation of the cxx library which does support passing closures with the inclusion and use of their cxx.h. However, autocxx inhibits its use and marks it as unsupported due to template specialization.

[adetaylor]

Yeah, sorry. I'd certainly be pleased to accept pull requests which can spot C++ functions that accept a cxx Fn and then pass that through all our various pipeline stages to feed the right sort of thing to cxx in our generated bindings. I suspect it might be tricky - I imagine the information we get from bindgen is incomplete or insufficient for this, but you never know. A good start would be to add a test to integration_tests.rs and then use these instructions to get logging out, paying particular attention to what information bindgen gives to autocxx at the start of the process.