src/output-proto.lisp, etc. (hook-output): Use standard combination.

src/output-proto.lisp, etc. (hook-output): Use standard combination.

This will let subclasses selectively override portions of the basic
output machinery.

The existing implicit traversal is now done via `:after' methods.
Existing hook methods which engaged custom traversal have been split out
into separate `:after' methods so that they don't get overridden by
accident.

The main output-class-to-header method (i.e., `hook-output' specialized
on `sod-class' and `(eql :h)') is huge and could usefully be split into
smaller pieces which can be overridden separately.