Reset world for determinism

[erincatto]

void b2World_ResetState( b2WorldId worldId )
{
    b2World* world = b2GetWorldFromId( worldId );
    B2_ASSERT( world->locked == false );
    if ( world->locked )
        return;

    // Contact Ids.
    {
        b2DestroyIdPool( &world->contactIdPool );
        world->contactIdPool = b2CreateIdPool();
    }

    // Broadphase Reset.
    // We might only need to do Kinematic and Dynamic?
    {
        b2BroadPhase* bp = &world->broadPhase;

        for ( int i = 0; i < b2_bodyTypeCount; ++i )
            b2DynamicTree_Destroy( bp->trees + i );

        for ( int i = 0; i < b2_bodyTypeCount; ++i )
            bp->trees[i] = b2DynamicTree_Create();
    }
}

From Melvyn. This would require a unit test.