Incorporate ConnyOnny's iterator exercise!

- Deciding not to have solutions in the tree right now, until I figure
  out a more general way to handle that.
- The assertions work really great as tests, actually!
- Simplifying a few of the side points-- I think they deserve their own
  exercises :)

README.md

@@ -108,6 +108,14 @@ The [Concurrency](https://doc.rust-lang.org/stable/book/concurrency.html) sectio
 
 - ["arc1.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+this+code+compile+by+filling+in+a+value+for+%60shared_numbers%60+where+the%0A%2F%2F+TODO+comment+is+and+creating+an+initial+binding+for+%60child_numbers%60%0A%2F%2F+somewhere.+Try+not+to+create+any+copies+of+the+%60numbers%60+Vec%21%0A%2F%2F+Scroll+down+for+hints+%3A%29%0A%0Ause+std%3A%3Async%3A%3AArc%3B%0Ause+std%3A%3Athread%3B%0A%0Afn+main%28%29+%7B%0A++++let+numbers%3A+Vec%3C_%3E+%3D+%280..100u32%29.collect%28%29%3B%0A++++let+shared_numbers+%3D+%2F%2F+TODO%0A++++let+mut+joinhandles+%3D+Vec%3A%3Anew%28%29%3B%0A%0A++++for+offset+in+0..8+%7B%0A++++++++joinhandles.push%28%0A++++++++thread%3A%3Aspawn%28move+%7C%7C+%7B%0A++++++++++++let+mut+i+%3D+offset%3B%0A++++++++++++let+mut+sum+%3D+0%3B%0A++++++++++++while+i+%3C+child_numbers.len%28%29+%7B%0A++++++++++++++++sum+%2B%3D+child_numbers%5Bi%5D%3B%0A++++++++++++++++i+%2B%3D+5%3B%0A++++++++++++%7D%0A++++++++++++println%21%28%22Sum+of+offset+%7B%7D+is+%7B%7D%22%2C+offset%2C+sum%29%3B%0A++++++++%7D%29%29%3B%0A++++%7D%0A++++for+handle+in+joinhandles.into_iter%28%29+%7B%0A++++++++handle.join%28%29.unwrap%28%29%3B%0A++++%7D%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Make+%60shared_numbers%60+be+an+%60Arc%60+from+the+numbers+vector.+Then%2C+in+order%0A%2F%2F+to+avoid+creating+a+copy+of+%60numbers%60%2C+you%27ll+need+to+create+%60child_numbers%60%0A%2F%2F+inside+the+loop+but+still+in+the+main+thread.%0A%0A%2F%2F+%60child_numbers%60+should+be+a+clone+of+the+Arc+of+the+numbers+instead+of+a%0A%2F%2F+thread-local+copy+of+the+numbers.%0A)
 
+#### Iterators
+
+Do not adjust your monitors-- iterators 1 and 2 are indeed missing. Iterator 3 is a bit challenging so we're leaving space for some exercises to lead up to it!
+
+Check out the [Iterators chapter of the book](https://doc.rust-lang.org/stable/book/iterators.html) and the [Iterator docs](http://doc.rust-lang.org/stable/std/iter/trait.Iterator.html).
+
+- ["iterator3.rs"](http://play.rust-lang.org/?code=%2F%2F+This+is+a+bigger+exercise+than+most+of+the+others%21+You+can+do+it%21%0A%2F%2F+Here+is+your+mission%2C+should+you+choose+to+accept+it%3A%0A%2F%2F+1.+Complete+the+divide+function+to+get+the+first+four+tests+to+pass%0A%2F%2F+2.+Uncomment+the+last+two+tests+and+get+them+to+pass+by+filling+in%0A%2F%2F++++values+for+%60x%60+using+%60division_results%60.%0A%2F%2F+Scroll+down+for+a+minor+hint+for+part+2%2C+and+scroll+down+further+for%0A%2F%2F+a+major+hint.%0A%2F%2F+Have+fun+%3A-%29%0A%0A%23%5Bderive%28Debug%2C+PartialEq%2C+Eq%29%5D%0Apub+enum+DivisionError+%7B%0A++++NotDivisible%28NotDivisibleError%29%2C%0A++++DivideByZero%2C%0A%7D%0A%0A%23%5Bderive%28Debug%2C+PartialEq%2C+Eq%29%5D%0Apub+struct+NotDivisibleError+%7B%0A++++dividend%3A+i32%2C%0A++++divisor%3A+i32%2C%0A%7D%0A%0A%2F%2F+This+function+should+calculate+%60a%60+divided+by+%60b%60+if+%60a%60+is%0A%2F%2F+evenly+divisible+by+b.%0A%2F%2F+Otherwise%2C+it+should+return+a+suitable+error.%0Apub+fn+divide%28a%3A+i32%2C+b%3A+i32%29+-%3E+Result%3Ci32%2C+DivisionError%3E+%7B%0A%7D%0A%0A%23%5Bcfg%28test%29%5D%0Amod+tests+%7B%0A++++use+super%3A%3A*%3B%0A%0A++++%2F%2F+Tests+that+verify+your+%60divide%60+function+implementation%0A++++%23%5Btest%5D%0A++++fn+test_success%28%29+%7B%0A++++++++assert_eq%21%28divide%2881%2C+9%29%2C+Ok%289%29%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+test_not_divisible%28%29+%7B%0A++++++++assert_eq%21%28%0A++++++++++++divide%2881%2C+6%29%2C%0A++++++++++++Err%28DivisionError%3A%3ANotDivisible%28NotDivisibleError%7B%0A++++++++++++++++dividend%3A+81%2C%0A++++++++++++++++divisor%3A+6%0A++++++++++++%7D%29%29%0A++++++++%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+test_divide_by_0%28%29+%7B%0A++++++++assert_eq%21%28divide%2881%2C+0%29%2C+Err%28DivisionError%3A%3ADivideByZero%29%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+test_divide_0_by_something%28%29+%7B%0A++++++++assert_eq%21%28divide%280%2C+81%29%2C+Ok%280%29%29%3B%0A++++%7D%0A%0A++++%2F%2F+Iterator+exercises+using+your+%60divide%60+function%0A++++%2F*%0A++++%23%5Btest%5D%0A++++fn+result_with_list%28%29+%7B%0A++++++++let+numbers+%3D+vec%21%5B27%2C+297%2C+38502%2C+81%5D%3B%0A++++++++let+division_results+%3D+numbers.into_iter%28%29.map%28%7Cn%7C+divide%28n%2C+27%29%29%3B%0A++++++++let+x+%2F%2F...+Fill+in+here%21%0A++++++++assert_eq%21%28format%21%28%22%7B%3A%3F%7D%22%2C+x%29%2C+%22Ok%28%5B1%2C+11%2C+1426%2C+3%5D%29%22%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+list_of_results%28%29+%7B%0A++++++++let+numbers+%3D+vec%21%5B27%2C+297%2C+38502%2C+81%5D%3B%0A++++++++let+division_results+%3D+numbers.into_iter%28%29.map%28%7Cn%7C+divide%28n%2C+27%29%29%3B%0A++++++++let+x+%2F%2F...+Fill+in+here%21%0A++++++++assert_eq%21%28format%21%28%22%7B%3A%3F%7D%22%2C+x%29%2C+%22%5BOk%281%29%2C+Ok%2811%29%2C+Ok%281426%29%2C+Ok%283%29%5D%22%29%3B%0A++++%7D%0A++++*%2F%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Minor+hint%3A+In+each+of+the+two+cases+in+the+match+in+main%2C+you+can+create+x+with+either+a+%27turbofish%27+or+by+hinting+the+type+of+x+to+the+compiler.+You+may+try+both.%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Major+hint%3A+Have+a+look+at+the+Iter+trait+and+at+the+explanation+of+its+collect+function.+Especially+the+part+about+Result+is+interesting.%0A)
+
 ### Threads
 
 See [the Dining Philosophers example](https://doc.rust-lang.org/stable/book/dining-philosophers.html) and the [Concurrency Chapter](https://doc.rust-lang.org/stable/book/concurrency.html) from the book.