Don't use deriveUnique *twice* in flattenTys.
Don't use deriveUnique *twice* in flattenTys.

Previously, we used deriveUnique and then uniqAway. This worked
doubly hard to avoid clashes. Doing just uniqAway is enough.

This commit also includes clarifying comments.

elieux/ghc,tjakway/ghcjvm,gcampax/ghc,GaloisInc/halvm-ghc,mettekou/ghc,tjakway/ghcjvm,TomMD/ghc,wxwxwwxxx/ghc,TomMD/ghc,sgillespie/ghc,ghc-android/ghc,vTurbine/ghc,oldmanmike/ghc,oldmanmike/ghc,ghc-android/ghc,elieux/ghc,nkaretnikov/ghc,ml9951/ghc,mfine/ghc,mettekou/ghc,sdiehl/ghc,vikraman/ghc,ghc-android/ghc,GaloisInc/halvm-ghc,shlevy/ghc,vTurbine/ghc,gcampax/ghc,oldmanmike/ghc,christiaanb/ghc,sdiehl/ghc,nushio3/ghc,olsner/ghc,nushio3/ghc,mettekou/ghc,sdiehl/ghc,acowley/ghc,ml9951/ghc,elieux/ghc,oldmanmike/ghc,nkaretnikov/ghc,tjakway/ghcjvm,mcschroeder/ghc,da-x/ghc,sdiehl/ghc,sgillespie/ghc,snoyberg/ghc,anton-dessiatov/ghc,mcschroeder/ghc,anton-dessiatov/ghc,TomMD/ghc,urbanslug/ghc,urbanslug/ghc,ezyang/ghc,ghc-android/ghc,GaloisInc/halvm-ghc,oldmanmike/ghc,acowley/ghc,gridaphobe/ghc,siddhanathan/ghc,AlexanderPankiv/ghc,sdiehl/ghc,tjakway/ghcjvm,anton-dessiatov/ghc,shlevy/ghc,vikraman/ghc,gridaphobe/ghc,ghc-android/ghc,gcampax/ghc,vikraman/ghc,mettekou/ghc,siddhanathan/ghc,olsner/ghc,elieux/ghc,shlevy/ghc,christiaanb/ghc,mfine/ghc,gridaphobe/ghc,snoyberg/ghc,sgillespie/ghc,sgillespie/ghc,ml9951/ghc,elieux/ghc,ezyang/ghc,mcschroeder/ghc,vikraman/ghc,mfine/ghc,gcampax/ghc,olsner/ghc,AlexanderPankiv/ghc,ezyang/ghc,anton-dessiatov/ghc,tjakway/ghcjvm,shlevy/ghc,christiaanb/ghc,sgillespie/ghc,mettekou/ghc,urbanslug/ghc,urbanslug/ghc,wxwxwwxxx/ghc,acowley/ghc,gcampax/ghc,anton-dessiatov/ghc,ml9951/ghc,mcschroeder/ghc,sdiehl/ghc,elieux/ghc,shlevy/ghc,wxwxwwxxx/ghc,nkaretnikov/ghc,mfine/ghc,fmthoma/ghc,ezyang/ghc,GaloisInc/halvm-ghc,olsner/ghc,siddhanathan/ghc,wxwxwwxxx/ghc,gcampax/ghc,urbanslug/ghc,TomMD/ghc,olsner/ghc,ghc-android/ghc,fmthoma/ghc,acowley/ghc,christiaanb/ghc,fmthoma/ghc,AlexanderPankiv/ghc,nkaretnikov/ghc,olsner/ghc,sgillespie/ghc,mettekou/ghc,vTurbine/ghc,da-x/ghc,da-x/ghc,mfine/ghc,sgillespie/ghc,wxwxwwxxx/ghc,da-x/ghc,urbanslug/ghc,TomMD/ghc,nkaretnikov/ghc,snoyberg/ghc,ml9951/ghc,snoyberg/ghc,oldmanmike/ghc,da-x/ghc,fmthoma/ghc,nushio3/ghc,elieux/ghc,sdiehl/ghc,nkaretnikov/ghc,vikraman/ghc,wxwxwwxxx/ghc,da-x/ghc,nkaretnikov/ghc,tjakway/ghcjvm,siddhanathan/ghc,siddhanathan/ghc,acowley/ghc,mcschroeder/ghc,vTurbine/ghc,siddhanathan/ghc,gridaphobe/ghc,ml9951/ghc,gridaphobe/ghc,shlevy/ghc,AlexanderPankiv/ghc,vikraman/ghc,snoyberg/ghc,ml9951/ghc,GaloisInc/halvm-ghc,mfine/ghc,ezyang/ghc,christiaanb/ghc,ml9951/ghc,nushio3/ghc,christiaanb/ghc,AlexanderPankiv/ghc,GaloisInc/halvm-ghc,acowley/ghc,TomMD/ghc,snoyberg/ghc,fmthoma/ghc,nushio3/ghc,gcampax/ghc,urbanslug/ghc,olsner/ghc,mettekou/ghc,AlexanderPankiv/ghc,anton-dessiatov/ghc,wxwxwwxxx/ghc,AlexanderPankiv/ghc,TomMD/ghc,vTurbine/ghc,anton-dessiatov/ghc,snoyberg/ghc,ezyang/ghc,tjakway/ghcjvm,vTurbine/ghc,oldmanmike/ghc,gridaphobe/ghc,vTurbine/ghc,acowley/ghc,christiaanb/ghc,mcschroeder/ghc,ezyang/ghc,shlevy/ghc,nushio3/ghc,ghc-android/ghc,fmthoma/ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,mcschroeder/ghc,da-x/ghc,gridaphobe/ghc,vikraman/ghc,nushio3/ghc,mfine/ghc,fmthoma/ghc