c_int, c_char, etc not supported as arguments

[danakj]

Similar to #592 but seems different.

I want to write an extern "C" function in cxx::bridge that receives an int or char or long or unsigned int etc. These types are represented in Rust with:

• https://doc.rust-lang.org/std/os/raw/type.c_char.html
• https://doc.rust-lang.org/std/os/raw/type.c_int.html
• etc.

However cxx produces an error that the type is not supported. So I end up falling back to writing wrapper functions on the C++ side with different types (like int32_t, uint8_t, or unsigned char), and generating bindings to them, but this is a big thorn for people trying to generate wrappers to typical C++ code.

[adetaylor]

Should be fixed by #874.