Better Backpack support with signature merging.

When we merge signatures, we gain exports that don't
necessarily have a source-level declaration corresponding
to them.  This meant Haddock dropped them.

There are two big limitations:

* If there's no export list, we won't report inherited
  signatures.

* If the type has a subordinate, the current hiDecl
  implementation doesn't reconstitute them.

These are probably worth fixing eventually, but this gets
us to minimum viable functionality.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
(cherry picked from commit 6cc832df)