Implement missing generators

Implement generators for the following exercises:

* [X] `hello-world` (#203)
* [X] `leap` (#212)
* [X] `collatz-conjecture` (#214)
* [X] `rna-transcription` (#218)
* [X] `bob` (#219)
* [X] `hamming` (#220)
* [x] `space-age`
* [x] `grains`
* [x] `nucleotide-count`
* [x] `difference-of-squares`
* [x] `triangle`
* [x] `sum-of-multiples`
* [x] `anagram`
* [x] `atbash-cipher`
* [ ] `phone-number`
* [x] `gigasecond`
* [x] `scrabble-score`
* [x] `two-fer`
* [x] `robot-simulator`
* [x] `word-count`
* [x] `clock`
* [x] `allergies`
* [x] `all-your-base`
* [x] `series`
* [x] `roman-numerals`
* [x] `sieve`
* [x] `largest-series-product`
* [x] `rotational-cipher`
* [x] `etl`
* [x] `luhn`
* [x] `meetup`
* [ ] `circular-buffer`
* [ ] `beer-song`
* [ ] `zipper`
* [x] `spiral-matrix`

* [X] `custom-set` (#222)
* [X] `complex-numbers` (#221)