Complain about ambiguity when using an overloaded field somewhere it shouldn't be
-XOverloadedRecordFields implies -XDisambiguateRecordFields
Lookup record selector names in local module only, avoiding bogus ambiguity errors
Report used RdrNames when looking up potentially overloaded variables
Tidy up code to create Has instances; ignore naughty record selectors
Maintain the AvailTC invariant when importing and exporting fields
Make FldParent for non-overloaded record fields in gresFromAvail