hamming: introduce use of @Rule

[jtigger]

After #142, the exercise hamming is now the first to use the JUnit @Rule feature. This feature is often not intuitive to those new-to-the-framework.

Please:

1. include a HINT.md file that describes the @Rule feature as succinctly and clearly as possible.
2. include a comment right about the use of @Rule that points the practitioner to JUnit documentation specifically about this feature (see QueenAttackCalculatorTest for inspiration).
3. Remove the inline comments that (re-)explain @Rule from tests that appear in exercises later in the track