`json_array` list initializes backwards

[Mogball]

json_array arr = {1, 2, 3, 4, 5};

The actual contents will be [5, 4, 3, 2, 1] because of the order in which template arguments unpack. I'd like to have a low runtime overhead way to reverse the arguments and have the sensible insertion order.

This seems promising, but I don't think we need something that complex, and we'd have to make sure wloop/wlib#142 is resolved