PR #3233 cleanup

- `::.head` became a `val`; excessive accessor removed
- SerializationProxy moved to `object List`
Fix typo in compiler's error message: anoynmous => anonymous
Merge pull request #3466 from pavelpavlov/typo-anoynmous

Fix typo in compiler's error message: anoynmous => anonymous
SI-8030 Restore thread safety to the parser

In addition to the invariant "parsing doesn’t enter new symbols",
which was respected and tested in 760df9843a910d6, we also must
ensure that "parsing doesn't call Symbol#info".

That happend indirectly if we call `companionModule`.

This commit just converts the name of `class TupleN` to a term
name, rather than getting the name of its companion.

No test is included. This is tested upstream in:

    https://jenkins.scala-ide.org:8496/jenkins/view/Memory%20Leak%20Tests/job/scalac-memory-leaks-test-2.11.0/
Merge pull request #3467 from retronym/ticket/8030-companion

SI-8030 Restore thread safety to the parser
Merge pull request #3465 from pavelpavlov/list-head-val

PR #3233 cleanup