robot-simulator: code && instructions is not matching

Hi,

I'm was trying to do the `robot-simulator` exercise. However, I found what was written in the README && what's in the test are very different.

While I appreciate a more complex & closer-to-life simulation, which was presented in the test,
it was not very clear what to do from the instructions alone.

What do you think about updating the exercise, such that:
- The tests and instruction match the [original specification](https://github.com/exercism/problem-specifications/tree/master/exercises/robot-simulator)
- Update the README && bonus test to add the extra conditions:
- Bounded room with wall (instead of infinite grid)
- Multiple robot being tested (instead of just one robot)

P.S: I also don't fully agree with some of the types && test defined, so I [made some changes](https://github.com/exercism/go/compare/master...exklamationmark:robot_simulator?expand=1)