Support outputting forward declarations

Cbindgen headers can get big. It may make sense to allow outputting a forward-declaration version of the header that declares the relevant types, though I haven't dug into how much effort it'd require.

Would this cover the case where I'd want to expose a struct only by pointer? E.g., I currently manually write:

struct sqlpe_state_machine;
int sqlpe_spsm_create(struct sqlpe_state_machine **spsm_ptr);
int sqlpe_spsm_free(struct sqlpe_state_machine *spsm_ptr);

If you'd consider that a separate idea, I'd gladly open a new issue.

That should already work afaict.