The current code uses std::stoi to convert the input string to an int
array_index. This limits the maximum addressable array size to ~2GB on
most platforms.

But all callers immediately convert the result of array_index to
BasicJsonType::size_type.

So let's parse it as unsigned long long, which allows us to have as
big arrays as available memory. And also makes the call sites nicer to
read.

One complication arises on platforms where size_type is smaller than
unsigned long long. We need to bail out on these if the parsed array
index does not fit into size_type.