[Feature Request] Box2D integration

[Trinkle23897]

https://github.com/openai/gym/tree/master/gym/envs/box2d

Env List:

• CarRacing-v1 (Box2D integration: Add CarRacing-v0 #71)
• BipedalWalker-v3 (Box2D integration: Add BipedalWalker, BipedalWalkerHardcore #131)
• BipedalWalkerHardcore-v3 (Box2D integration: Add BipedalWalker, BipedalWalkerHardcore #131)
• LunarLander-v2 (Box2D integration: Add LunarLander #111)
• LunarLanderContinuous-v2 (Box2D integration: Add LunarLander #111)

Road Map:

• Get comfortable with current codebase, go through https://envpool.readthedocs.io/en/latest/pages/env.html and add a toy environment by yourself locally;
• Run Box2D environments on your local machine [2], try with different env settings and see the actual behavior;
• Go through pyBox2d code [1], think about how can we directly call those methods via EnvPool single environment abstraction;
• Integrate only one game and let it work (you only need to translate python to C++);
• Add some unit tests (good to submit the first PR here);
• Integrate other environments (submit another PR) and related tests.

Resources:

1. https://github.com/pybox2d/pybox2d/tree/master/Box2D
2. First install gym, then run with

import gym
env = gym.make("CarRacing-v0")
env.reset()
for _ in range(10):
  env.step(env.action_space.sample())
  env.render()