Tidyup - previous instance-decl commit fell a bit short:

 * RnEnv.lookupInstDeclBndr unceremoniously fell over when passed
   an out-of-scope class name.

 * the AvailEnv carried around didn't common up type/class info
   (i.e.,  AvailTCs), but rather type/class and method/label names,
   causing the renamer to (semi)randomly report instance methods as
   being out-of-scope in the presence of multiple imports for a module.

 * didn't support 'hiding' of class / method names (for the purposes
   of checking instance decls).