Improvements to util/nest.py and data/util/nest.py

Changes:
- Add a cache for type -> is_sequence to speed up Flatten/IsSequence
- Update data/util/nest.py flatten to use C Flatten

Before:
entry {
  name: "EagerLinearRegressionBenchmark.eager_train_cpu"
  iters: 2000
  wall_time: 1.91852378845
  extras {
    key: "examples_per_sec"
    value {
      double_value: 66717.9634521
    }
  }
}

After:
entry {
  name: "EagerLinearRegressionBenchmark.eager_train_cpu"
  iters: 2000
  wall_time: 1.74479198456
  extras {
    key: "examples_per_sec"
    value {
      double_value: 73361.1806638
    }
  }
}
PiperOrigin-RevId: 197497854