code: function int64add(dst, x, y) {
          var w0 = (x.l & 0xffff) + (y.l & 0xffff);
          var w1 = (x.l >>> 16) + (y.l >>> 16) + (w0 >>> 16);
          var w2 = (x.h & 0xffff) + (y.h & 0xffff) + (w1 >>> 16);
          var w3 = (x.h >>> 16) + (y.h >>> 16) + (w2 >>> 16);
          dst.l = (w0 & 0xffff) | (w1 << 16);
          dst.h = (w2 & 0xffff) | (w3 << 16);
        }
docs: Adds two 64-bit numbers Like the original implementation, does not rely on 32-bit operations