day04 input parsing will panic for some violations

[Bodobolero]

Hi Tim, I admire your elegant, idiomatic solutions, however sometimes you got lucky.

For example in day04 parsing at

advent-of-code-2020/day04b/src/main.rs

Line 26 in 0dd2865

"byr" => value.parse::<usize>().unwrap().wrapping_sub(1920) <= 82,

you would panic due to calling unwrap() in case the token following byr: is not a sequence of 4 digits (which the task description asked us to check):

https://adventofcode.com/2020/day/4

You can continue to ignore the cid field, but each other field has strict rules about what values are valid for automatic validation:

byr (Birth Year) - four digits; at least 1920 and at most 2002.

Example input:

hgt:157cm byr:hugo ecl:grn iyr:2012
eyr:2030 hcl:#18171d pid:173cm

(Just stumbled upon your solution when trying to improve my Rust coding style to become more idiomatic and looking at other people's solutions)