format, use util vec2 in netbook days
Hook up netbook days, download all remaining inputs
2022: Add days 13, 14 (partial) 18, 21, 23, which were done on a netbook with Go 1.15
2022: Factor out vec2, format utils
2022: Fix d12

Unsigned types are evil (underflow)
2022: nonworking d12
2022: d11p2 attribution
2022: d11p2

Insert generic 'I hate math' comment
Store test mod explicitly
2022: finish d9
2022: d9: prep for p2
2022: significantly beautify d8 and update test expectations
2022: Clean up d7