The list elements are already being allocated out of a
single memory buffer.  We can drop the Link* pointer
following and the memory it requires, replacing it with
index operations.

The change also keeps a channel from containing a pointer
back into its own allocation block, which would create a
cycle.  Blocks involved in cycles are not guaranteed to be
finalized properly, and channels depend on finalizers to
free OS-level locks on some systems.  The self-reference
was keeping channels from being garbage collected.

runtime-gdb.py will need to be updated in order to dump
the content of buffered channels with the new data structure.

Fixes #1676.

R=ken2, r
CC=golang-dev
https://golang.org/cl/4411045