Add flatten-array

[BNAndras]

@BethanyG, this is another one for your review, but it's not a priority since we have enough exercises for launch.

I set this up using lists for simplicity, but I'm thinking the exercise could benefit from having the students deal with mutable [arrays][https://pyret.org/docs/latest/arrays.html] instead. We don't have any exercises yet that lend themselves to an array since lists are more versatile. If we swap to arrays, a student can still use lists internally as long as they convert the list back to an array at the end. If they use arrays, they'd likely need a [for loop[https://pyret.org/docs/latest/A_Tour_of_Pyret.html#%28part._.For_loops%29] which might bump up the difficulty. Either way, we'll want an instructions append to clarify to students what we're expecting since the instructions reference lists in an exercise whose name references arrays.