runtime: reusable intrusive doubly-linked list

runtime: reusable intrusive doubly-linked list

Unfortunately we have two nearly identical types. One for standard types
and one for sys.NotInHeap types or cases that must avoid write barriers.
The latter must use uintptr fields, as assignment to unsafe.Pointer
fields generates a write barrier.

Change-Id: I6a6a636c62d83fa93b991033c7108d3b934412ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/714020
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>