x/debug: change various peeking functions to be methods of Server instead of Printer so they can be used elsewhere.

Make them return error values instead of a bool that indicates success.

Move these functions to peek.go.

Change-Id: Ifbd198ccd162e49e277c7a3b927abe47d18c6955
Reviewed-on: https://go-review.googlesource.com/11881
Reviewed-by: Rob Pike <r@golang.org>