debug: Add a function for computing the stride of an array.

Make sizeof() return a bool indicating success.

Continue printing slices even if we can't determine the stride.

Ran go fmt.

Change-Id: I9cf3ae3806223dd7b8b420c817f02900fe7a1a05
Reviewed-on: https://go-review.googlesource.com/2684
Reviewed-by: Rob Pike <r@golang.org>