x/debug: support getting values of channel type.

program.Channel has a method Element, similar to that of array and
slice, that allows reading of the channel's buffered values.

Change-Id: Ic62d3319f231a685035c36a01f6c5f0b8fcfd87c
Reviewed-on: https://go-review.googlesource.com/13880
Reviewed-by: Rob Pike <r@golang.org>