Added slope() function to HorizontalTerrain and VerticalTerrain
Added x_at() and y_at() to HorizontalTerrain and VerticalTerrain respectively
Refactoring (slope() call)
Added a `prev` property to `Physical` and update it accordingly
Bug fix: `CollisionMaster` was not calling the correct collision functions for HorizontalTerrain and VerticalTerrain
Bug fix: `Physical.prev` did not have an `anchor` property like it's parent thingy
Bug fix: when player was stepping on corners, collisions weren't right
Filled in functionality to `CollisionMaster._do_collide_physical_horizontal()`
Removed TODO that was already todone