add support for ctypes 0.23

[ch1y1z1]

ctypes 0.23 changed view of string (ptr (const char)), when string appears in external function arg, there will be an exception:

external%c func: s_:string -> void = {|
// $s_
return;
|}

Fatal error: exception File "src/internal/ppxc__script_real.ml", line 449, characters 6-11: Pattern matching failed