Refactor (again) the handling of default methods

Refactor (again) the handling of default methods

This patch fixes Trac #4056, by

 a) tidying up the treatment of default method names
 b) removing the 'module' argument to newTopSrcBinder

The details aren't that interesting, but the result
is much tidier. The original bug was a 'nameModule' panic,
caused by trying to find the module of a top-level name.
But TH quotes generate Internal top-level names that don't
have a module, and that is generally a good thing.

Fixing that in turn led to the default-method refactoring,
which also makes the Name for a default method be handled
in the same way as other derived names, generated in BuildTyCl
via a call newImplicitBinder.  Hurrah.