Rng.uniform,gaussian,next returns a Stream now

[rainyl]

make Rng.uniform, next, gaussian and async version of them return Stream

final rng = cv.Rng();
final v = await rng.uniformAsync(0, 241, maxCount: 100000).take(10000).toList();
expect(v, everyElement(greaterThanOrEqualTo(0)));
final v1 = await rng.uniformAsync(2.41, 241.0).take(100000).toList();
expect(v1, everyElement(greaterThanOrEqualTo(2.41)));

final v = await rng.gaussian(2.41).take(100000).toList();
expect(v.length, equals(100000));

final v1 = await rng.next().first;
expect(v1, isA<int>());

final rng = await cv.RngAsync.createAsync();
final v = await rng.uniformAsync(0, 241, maxCount: 10000).take(10000).toList();
expect(v, everyElement(greaterThanOrEqualTo(0)));
final v1 = await rng.uniformAsync(2.41, 241.0).take(10000).toList();
expect(v1, everyElement(greaterThanOrEqualTo(2.41)));
final v = await rng.gaussianAsync(2.41).take(10000).toList();
expect(v.length, equals(10000));

final v1 = await rng.nextAsync().first;
expect(v1, isA<int>());