Implement OSLICE, OSLICEARR, OSLICESTR, OSLICE3, OSLICE3ARR.

reviewer: Ignore the code in OINDEX, that's from CL 14466.

Change-Id: I00cc8aecd4c6f40ea5517cd660bb0ce759d91171
Reviewed-on: https://go-review.googlesource.com/14538Reviewed-by: Todd Neal <todd@tneal.org>