Keep Plists in a slice instead of a linked list.
Eliminate unnecessary fields.
Also, while here remove gc's unused breakpc and continpc vars.

Change-Id: Ia04264036c0442843869965d247ccf68a5295115
Reviewed-on: https://go-review.googlesource.com/29367
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>