feature: makes "compile" exercise print output, resolves #270

When running "compile"-mode exercises in interactive `verify` mode,
we print their output when we prompt the learner if they want to
continue. This improves the "experimentation" experience, since
trying different things does produce a visible change.

exercises/primitive_types/primitive_types4.rs

@@ -8,7 +8,4 @@
 fn slice_out_of_array() {
     let a = [1, 2, 3, 4, 5];
 
-    let nice_slice = ???
-
-    assert_eq!([2, 3, 4], nice_slice)
 }