Need to record types for callable()

[DavisVaughan]

The generated callable forward declaration is wrong for callable() lines because it assumes they all have SEXP arguments. If we record the types while parsing we can use them here to generate the correct declaration https://github.com/DavisVaughan/vctrs/blob/69f839f41975871edfd40604b71ea44938f9061a/src/init.c#L188

[DavisVaughan]

Maybe work in reverse for each argument. Find the first space " " from the back. Check that * is not beside the variable name, and then assume that everything else before that space is the argument type (maybe adding back in * as required). This would allow less common types to work too, like enum vctrs_enum_thing arg