Allow discrete space to be iterable

[EugenHotaj]

It seems intuitive that spaces should be iterable (particularly discrete spaces). For example, if this represents an action space one could iterate as follows:

for action in actions:
...

instead of:

for action in range(actions.n):

[geyang]

Good idea, non-ideal stateful implementation.

[pzhokhov]

this feels vaguely unnecessary... I am not sure statement for action in action_space: is more lucid than for action in range(action_space.n). In order for this to make sense, I'd like two things:

1. examples of use cases for box (the iterator can return a random walk)
2. as @episodeyang pointed out, stateless implementation (for instance iterable pattern instead of an iterator, i.e. an iter() method that returns a stateful iterator). Closing this, feel free to reopen with respective changes.