lookupFunction<T extends Function, F extends Function> method Null safety

Looks up a native function and returns it as a Dart function.

T is the C function signature, and F is the Dart function signature. For example:

int32_t add(int32_t a, int32_t b) {
  return a + b;
}

DynamicLibrary dylib;
final add = dylib.lookupFunction<Int32 Function(Int32, Int32),
                                 int Function(int, int)>('add');