SI-8143 Regressions with override checks, private members

These regressed in e609f1f20b, which excluded all private methods from
overriding checks. We should only exclude private[this] members on the
low end of a pair, as was done before that commit, and, we must also
exclude private members on the high side.

Why? Warning: reverse engineered intuition follows.

We need to report an error when if a private method in a subclass
has matches a less-private method in the super class and report an
error, lest the user be fooled into thinking it might be invoked
virtually. On the other hand, adding a private method to a super
class shouldn't invalidate the choice names of public members in
its superclasses.

I've removed the test case added by that commit and will lodge a
reworked version of it that Paul provided as a new issue. That shows
a bug with qualified private + inheritance.

In addition, the expectation of `neg/accesses.check` is reverted
to its 2.10.3 version, which I believe is correct. When it was
changed in e609f1f20b it sprouted a variation, `neg/accesses-2`,
which has now changed behaviour. The intent of that test will
be captured in the aforementioned issue covering qualified private
inheritance.
SI-7700 @unspecialized, Part Deux: Now Working.

This annotation was introduced to allow us to mark methods within
a specialized trait as immune from specialization. In particular,
this is desirable for `Function1.{andThen, compose}`.

However, it seems we need to check for this in two places in the
specialization code base. The feature is now backed with a test.
Merge pull request #3357 from retronym/ticket/8143

SI-8143 Regressions with override checks, private members
Merge pull request #3425 from retronym/ticket/7700

SI-7700 @unspecialized, Part Deux: Now Working.