x/debug: support getting values of slice type.

The type program.Slice that the user gets for slice variables just
embeds a program.Array and adds a capacity field.

The DWARF information for slices doesn't include stride, so the stride
is computed with the size of the element type.

Change-Id: I69c60998abd1b48b1986522f60f93ddcad0c155e
Reviewed-on: https://go-review.googlesource.com/12831
Reviewed-by: Rob Pike <r@golang.org>