Remove wasn't nil'ing the *Element.id. This property was exploited
by MoveToFront and MoveToBack internally, so I renamed the existing
Remove to "remove", and created an exported wrapper "Remove" that does
the right thing for the user's sake.

Also, saved an allocation by using *List as the id rather than *byte.

Fixes #1224.

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/2685042