Merge branch 'kdoc-item2' into docs-mw

The kerneldoc parsing phase gathers all of the information about the
declarations of interest, then passes it through to the output phase as a
dict that is an unstructured blob of information; this organization has its
origins in the Perl version of the program.  It results in an interface
that is difficult to reason about, dozen-parameter function calls, and
other ills.

Introduce a new class (KdocItem) to carry this information between the
parser and the output modules, and, step by step, modify the system to use
this class in a more structured way.  This could be taken further by
creating a subclass of KdocItem for each declaration type (function,
struct, ...), but that is probably more structure than we need.

The result is (I hope) clearer code, the removal of a bunch of boilerplate,
and no changes to the generated output.