container/list: Correctly maintain internal invariants

The previous implementation was a mess with invariants
maintained inconsistently. Essentially reimplemented
the package:

- used a circular list as internal representation for
  significantly simpler implementation with fewer
  special cases while maintaining the illusion of
  a nil-terminated doubly linked list externally

- more precise documentation

- cleaned up and simplified tests, added test case
  for issue 4103.

No changes to the API or documented semantics.

All this said, I would be in favor of removing
this package eventually. container/ring provides
a faster implementation and a simpler and more
powerful API.

Fixes #4103.

R=r
CC=golang-dev
https://golang.org/cl/6569072