Use `unimplemented!` to reference variables, instead of `allow(unused_variables)`

[petertseng]

We desire for our stubs to compile without warnings, as that helps us set a better example and prevent problems like #461 (a stub that provides a u32 where a u64 is needed).

In order for us to get there, these files that have allow(unused_variables) in them should stop using allow(unused_variables) and instead use the model established in #438 #474 and #477, where in the unimplemented! macro we write a message saying what the function should do and reference the arguments.

Here are the files that should be modified as a result of this:

$ git grep unused_variables
exercises/all-your-base/src/lib.rs:#[allow(unused_variables)]
exercises/grade-school/src/lib.rs:#[allow(unused_variables)]
exercises/ocr-numbers/src/lib.rs:#[allow(unused_variables)]
exercises/react/src/lib.rs:#[allow(unused_variables)]
exercises/robot-simulator/src/lib.rs:    #[allow(unused_variables)]
exercises/robot-simulator/src/lib.rs:    #[allow(unused_variables)]
exercises/space-age/src/lib.rs:#![allow(unused_variables)]

You may choose to either work on these one at a time, or send one PR that does this for all the exercises.