Generate a C header file to match the C api

[Marwes]

The C api won't be very useful without a C header with all the functions and types defined and while this could be created and maintained manually that forces some duplication. Instead we may want to generate this manually with something like rusty-cheddar (there may be other alternatives as well but I am not aware of any).

Marking this as beginner as it should require only light knowledge about gluon's internals.

[memoryruins]

cbindgen appears to be the actively worked on solution for bindings these days, developed alongside servo's WebRender. Supports generics and C++ output as well.