General - Just keep the last time the number was spoken around and then look it up when you need to.

Note: For debugging I keep the list of turns a number was spoken around and I also build/keep the list of ages. Might need to optimize this for part2 (to reduce the mem/stack size).

Part 1 - Take 2020 turns and return the last number spoken.

Part 2 - Take 30000000 turns and return the last number spoken. Big surprise (or not): The implementation for part1 is slow, but ... ... not so slow that it is not returning a solution within 2 minutes.