hamming: Lack of type signature in stub solution

[rbasso]

In the past we didn't had stub solutions in src.

When we added the stubs to all exercises, we didn't added type signatures to some of the exercises that could have multiple types, to avoid "choosing" one for the students.

After a while, we started adding signatures and changing some exercises adding HINTS.md files, explaining that they could accept multiple types.

We should consider doing the same withhamming!

#181
#182
#431
#393