Stand the Human
Improve the data design

- Entity has [BodyData].
- BodyData has [FixtureData].
- BodyData has b2Body.
- FixtureData has b2Fixture.

- BodyData and FixtureData remain on the memory after the b2Body or the
  b2Fixture are destroyed.
- BodyData and FixtureData leave the list of their owner
  (Entity.bdata_dict and BodyData.fdatas) at their leave_game().