Replace section 4.1 with section A.3.

The new section 4.1 is titled "Naming and binding."  It includes the
text of section A.3 augmented with some of the detailed text from the
old section 4.1.

The \dfn, \index stuff is probably wrong, but I tried.

Also update other parts of appendix A to mention that nested scopes
and generators are standard features.