Refactor error expansion to locate errors in koan module

The previous strategy wasn't robust enough to detect typos in function
calls. For example, if you misremembered String.duplicate/2 as
String.repeat/2 the koan runner would hang and have to be restarted.
This is because the stacktrace for an error like that doesn't have line
numbers. To avoid the problem, we detect the first pieces of the
stracktrace that is in the koan module. I can't decide if that's a
perfect solution, but it seemed to work in my testing. Hoping to get
other's feedback.

lib/execute.ex

@@ -32,22 +32,21 @@ defmodule Execute do
 
   defp exec(module, name, args, parent) do
     result = apply(module, name, args)
-    send parent, expand(result)
+    send parent, expand(result, module)
     Process.exit(self(), :kill)
   end
 
-  defp expand(:ok), do: :ok
-  defp expand(error) do
+  defp expand(:ok, _), do: :ok
+  defp expand(error, module) do
     {file, line} = System.stacktrace
-      |> Enum.drop_while(&in_ex_unit?/1)
+      |> Enum.drop_while(&!in_koan?(&1, module))
       |> List.first
       |> extract_file_and_line
 
       %{error: error, file: file, line: line}
   end
 
-  defp in_ex_unit?({ExUnit.Assertions, _, _, _}), do: true
-  defp in_ex_unit?(_), do: false
+  defp in_koan?({module, _, _, _}, koan), do: module == koan
 
   defp extract_file_and_line({_, _, _, [file: file, line: line]}) do
    {file, line}