Add an optional wiki link for each top level exported name.

In each module, for each "top level" exported entity we add a hyper link to a
corresponding wiki page. The link url gets the name of the exported entity as
a '#'-style anchor, so if there is an anchor in the page with that name then
the users browser should jump directly to it. By "top level" we mean functions,
classes, class members and data types (data, type, newtype), but not data
constructors, class instances or data type class membership.

The link is added at the right of the page and in a small font. Hopefully this
is the right balance of visibility/distraction.

We also include a link to the wiki base url in the contents and index pages.