Add more warnings and errors.

[Sean1708]

But only when it makes sense.

For example a public C function which is not #[no_mangle] is likely that way because the author forgot to include it. On the other hand, a public struct is quite likely to be not intended to be used from C so we don't want to warn about that.

Combinations which should be warned:

• pub extern <C style ABI> fn with no #[no_mangle]
• #[no_mangle] pub fn with no extern
• use of types which are not pointers, function pointers, or paths

[Sean1708]

• Anything marked #[repr(C)] should be public and C-calllable
  • no generics
  • no unit structs
  • single unit tuple structs only (opaque pointers)
• Anything marked #[no_mangle] should be public and external
• Anything external should be public and #[no_mangle]
• Any functions pointers should be extern
  • maybe this should only be applied when in a struct or function args