Remove old code for linking class members source.

@class now always gets split up to multiple docsets, so there's no
need to loop through the members of a class docset, as there will never
be any.
Move creation of :files field from File to Merger.

This way the Tag#merge method will have access to both the filename and linenr,
which is useful for error reporting.

Previously the Merger introduced :linenr field, which was largely unused, except
for some tests checking it.  This resulted in exported JSON containing both :linenr
and :files fields - but the latter already has :linenr inside it.  So another good
step towards more consistency.
Warn when less params documented than detected from code.
More explicit/implicit param mismatch warnings.
Disable the param_count warning by default.