Advent of Nim 2020 megathread

[e]

I needed a hint for Day 23, and went to reddit where I saw the SPOILER from 
[u/3urny](https://www.reddit.com/user/3urny/) "use a flat integer array of 
ints, length 1M, where at index at n you put which number comes after n in the 
ring." So, no linked list, no hash table. [My 
solution](https://github.com/dcurrie/AdventOfCode/blob/master/AoC_2020/23/p2.nim)
 is about 32 lines if you remove the commented out debugging code.