Draft: Change T_int to long so we can use larger arrays.

Changed T_int, Integer (ast) to long so we can index arrays larger than 16 GiB. The purpose of this branch is not to be merged per se, but to chase the C compiler down the rabbit hole. With -Wall -Werror, it will point exactly to the places where C-code interfaces with the T_int and Integer types, so it becomes trivial, but time-consuming to fix. Once that is done, we can put the T_int back to int and use the diff to indentify the places where we need to replace it by T_long.