Implement missing generators

[NobbZ]

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)

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.