Pass FnPtr params as ffi.Callback

[bengreenier]

Sometimes we may wish to use ffi.Callback to represent a function pointer that's being passed to a native function as a parameter. Today, these types get mapped as ffi.Function which isn't compatible. We should consider how to support a typed solution that allows either:

• Select ffi.Function or ffi.Callback via clangffi flags or other user config (ideally selection occurs at a function level)
• Provide a typing that allows for either, and let the caller decide

[bengreenier]

as a workaround today, one may pass a ffi.Callback() anywhere a ffi.Function() is taken as a param, but they must cast it to any.