ffigen should return enums, not ints

[Levi-Lesches]

Related to #446. Given the following C code:

typedef enum {
  ok = 1,
  error = 2,
} Status;

Status getStatus() { return ok; }

int getValue(Status status) {
  if (status == ok) return 1;
  else return 2;
}

ffigen will generate the following Dart code:

enum Status {
  ok(1),
  error(2);

  final int value;
  const Status(this.value);
}

// ...

int getStatus() {
  return _getStatus();
}

int getValue(int status) {
  return _getValue(status);
}

So it's still returning an integer. I think it should instead generate something like:

enum Status {
  // ...
  static Status fromValue(int value) => switch (value) {
    1 => Status.ok,
    2 => Status.error,
    _ => throw ArgumentError("Invalid value for Status: $value"),
  };
}

// ...

Status getStatus() {
  return Status.fromValue(_getStatus());
}

int getValue(Status status) {
  return _getValue(status.value);
}

cc @dcharkes and @eernstg from the previous discussions. PR at #1202