Changing the behaviour of 'walk'

- There is a new type for Action
- There is a distinction now between the state that 'walk' updates
	and the state that the action keeps track of / updates
- Walk still calls the action at each call
- Walk however doesn't know about the type of the state that the
	action updates, it only passes it along and finally returns it
- nsteps has also been removed, walk now takes one step at a time
	- A separate (meta-)action could be that of executing an action
		every n steps
- Tests have not been updated, so those will not work for now