SCC analysis for instances as well as types/classes

This big patch is in pursuit of Trac #11348.

It is largely the work of Alex Veith (thank you!), with some
follow-up simplification and refactoring from Simon PJ.

The main payload is described in RnSource
  Note [Dependency analysis of type, class, and instance decls]
which is pretty detailed.

* There is a new data type HsDecls.TyClGroup, for a strongly
  connected component of type/class/instance/role decls.

  The hs_instds field of HsGroup disappears, in consequence

  This forces some knock-on changes, including a minor
  haddock submodule update

Smaller, weakly-related things

* I found that both the renamer and typechecker were building an
  identical env for RoleAnnots, so I put common code for
  RoleAnnotEnv in RnEnv.

* I found that tcInstDecls1 had very clumsy error handling, so I
  put it together into TcInstDcls.doClsInstErrorChecks