Support -k for arrays

[Harvie]

Hi...

I just expect -k on ["a","b","c"] to output:
0
1
2
instead of error... it will make loops in bash easy... now i have to get "3" using -l, subtract 1 to get 2 and use "seq 0 2" to drive for loop properly...

Thx

[keenerd]

What is your use case? I'm guessing it can probably be done much faster and more simply by using -a instead.