Add my-length.
Revert "Add my-length."

This reverts commit 78cdcf8d9dbe9c213ade6681ad918110a80c871b.
Revert "Revert "Add my-length.""

This reverts commit 57f20212e59b2fab910e8c01a1c39eccabc92821.
Add my-length: update koans.