https://adventofcode.com/2015/day/12

Sum up the numbers in a JSON value.

Rather than pull in a heavy JSON parsing dependency, this just parses out the subset of JSON that the problem uses.

>>> :main + "[1,2,3]\n"
6
6

>>> :main + "{\"a\":2,\"b\":4}\n"
6
6

>>> :main + "{\"a\":{\"b\":4},\"c\":-1}\n"
3
3

>>> :main + "[1,{\"c\":\"red\",\"b\":2},3]\n"
6
4

>>> :main + "{\"d\":\"red\",\"e\":[1,2,3,4],\"f\":5}\n"
15
0

>>> :main + "[1,\"red\",5]\n"
6
6