🐛 bug: i. sequences should start from zero

[cratelyn]

this fixes a small bug for the i. operator, which was generating sequences 1..n, rather than 0..n-1.

comparing with the official J runtime for reference:

; jconsole
   i. 0

   i. 5
0 1 2 3 4

   i. 5 5
 0  1  2  3  4
 5  6  7  8  9
10 11 12 13 14
15 16 17 18 19
20 21 22 23 24

; cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/j`
  i. 0

  i. 5
0 1 2 3 4

  i. 5 5
0 1 2 3 4
5 6 7 8 9
10 11 12 13 14
15 16 17 18 19
20 21 22 23 24