dwrap0<R> method
Binds a cwraped C function taking 0 arguments and returning a plain
(non-struct) value.
R determines the JS->Dart return conversion applied on every call
(see _conv); use void if the C function returns nothing.
Implementation
WF0<R> dwrap0<R>(String name) {
final f = _wn(name, 0);
final c = _conv<R>();
return () => c(f.run());
}